Double-beam light source apparatus, position detecting apparatus and aligning apparatus

ABSTRACT

A position detecting apparatus comprises a double-beam producing device for producing two beams different in frequency from each other, which are guided to irradiate a diffraction grating on an object to be inspected in two predetermined directions, and a detector photoelectrically detecting through an objective optical system diffracted light produced by the diffraction grating, in which the double-beam producing device comprises a light source for supplying a beam of a single wavelength or multiple wavelengths, a beam splitting device for splitting the beam from the light source into two predetermined beams, a relay optical system for converging the two split beams at a predetermined position, and a frequency difference producing device disposed at or near a converging position by the relay optical system, for producing a predetermined frequency difference between the two split beams.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a double-beam light source apparatussuitable for position detection of heterodyne type and a positiondetecting apparatus of heterodyne type using the double-beam lightsource apparatus, and is suitably applicable specifically to ahigh-precision aligning apparatus for aligning a wafer or mask insemiconductor production equipment.

2. Related Background Art

Recently, projection exposure apparatus, which are so called steppers,are frequently used as apparatus for replication of fine pattern forexample for semiconductor elements onto a wafer of semiconductor at ahigh resolution. In particular, since a recent demand is an increase indensity of LSI produced by such apparatus, a finer pattern is desired tobe replicated onto a wafer. Higher precision positioning (alignment) isnecessary for replication of finer pattern.

For example, Japanese Laid-open Patent Application No. 62-261003discloses an apparatus for carrying out high-precision positiondetection using the heterodyne interference method.

This apparatus utilizes a Zeeman laser, which emits a beam includingP-polarized light and S-polarized light having respective frequenciesslightly different from each other, as a light source for alignment, inwhich the beam from the Zeeman laser is split by a polarized beamsplitter into P-polarized light of frequency f₁ and S-polarized light offrequency f₂ and the thus split beams are guided via a reflection mirrorto irradiate a diffraction grating mark (alignment mark) formed on areticle (mask) in two predetermined directions. A transparent window isprovided at a position adjacent to the diffraction grating mark on thereticle, through which a part of the beams impinging on the diffractiongrating mark pass to irradiate a diffraction grating mark formed on awafer in two predetermined directions.

When the two beams different in frequency from each other thus irradiatethe diffraction grating marks respectively in the two directions,diffracted light from each diffraction grating mark is made to passthrough a polarizer in detection system as to interfere with each other,whereby two,optical beat signals are obtained by photoelectricconversion of interference light by respective photoelectric detectors.A relative phase difference between the two signals corresponds to adeviation amount between the two beams crossing each other ondiffraction grating marks and the substrate (reticle or wafer). Forexample, with a reference signal, which is either one of the opticalbeat signals detected, the reticle is relatively moved to the wafer suchthat the phase difference becomes zero or a certain value, wherebyhigh-precision position detection is carried out.

It is, however, difficult for the position detecting apparatus disclosedin Japanese Laid-open Patent Application No. 62-261003 to perfectlyseparate the P-polarized light from the S-polarized light. For example,the beam of frequency f₂ could be mixed in the beam of frequency f₁originally expected to be pure to irradiate the diffraction gratingmarks, which results in degrading the SN ratio of optical beat signalsobtained. The degradation of SN ratio raises a problem of lowering thedetection precision.

Japanese Laid-open Patent Application No. 2-227604 discloses anotherposition detecting apparatus which can perform position detection atexcellent SN ratio, using the heterodyne interference method.

This apparatus is so arranged that a beam from laser source is splitinto two beams by a beam splitter and that one of the two split beams ismade to pass through one of two different acousto-optic modulators (AOM)and the other beam through the other AOM, whereby the two beams haverespective frequency differences different from each other. The twobeams having the frequency differences different from each otherirradiate diffraction grating marks on reticle and on wafer respectivelyin two directions. Diffracted light components going out of thediffraction grating marks in the same direction are made interfered witheach other. Two optical beat signals are obtained by photoelectricconversion of interference light by respective photoelectric detectors.Relative alignment is achieved at high precision between reticle andwafer using the two optical beat signals. As so arranged, the mixture ofbeams different in frequency can be avoided so as to enable detectionwith excellent SN ratio.

In the position detecting apparatus employing the heterodyneinterference method as disclosed in above Japanese Laid-open PatentApplication No. 2-227604 there are, however, used an optical member(including the beam splitter) for separating a beam from laser source toproduce two beams mutually different in frequency (heterodyne beams) andtwo acousto-optic modulators for producing the frequency differences inthe two beams leaving the optical member. Such arrangement iscomplicated and increases the scale of apparatus, which wasinconvenient. Also, a first problem was that the adjustment of opticalmember was difficult and it was therefore too difficult to keep theprecision of position detection within a certain permissible error rangein the arrangement in which the two beams different in,frequency areproduced by provision of two acousto-optic modulators.

Also, in the position detecting apparatus employing the heterodyneinterference method as disclosed in above Japanese Laid-open PatentApplication No. 2-227604, an optical path difference between the twosplit beams increases in proportion to wavelength if the two beams(heterodyne beams) different in frequency from each other are producedby splitting a beam from laser source. Therefore, it is theoreticallyinevitable to use monochromatic light (light of single wavelength), suchas laser beam, as the light for position detection. If the alignment iscarried out with monochromatic light before replication of circuitpattern on reticle onto a wafer with a resist (photosensitive material)deposited thereon, a second problem of lowering the alignment precisionwill arise from influence of thin film interference by the resist.

Generally, as a wafer goes through multiple processes, its mark foralignment tends to collapse in cross section, resulting in the crosssection being asymmetric. Under such circumstances, the alignment methodusing interference of monochromatic light such as laser beam had a thirdproblem of lowering the detection precision of position of mark foralignment as the cross section of alignment mark becomes moreasymmetric.

SUMMARY OF THE INVENTION

The present invention has been accomplished taking the above problemsinto consideration. It is a first object of the present invention toprovide a double-beam light source apparatus suitable for heterodyneinterference method, solving the first problem, and to provide aposition detecting apparatus utilizing the heterodyne interferencemethod, which is relatively simple in structure and easy in adjustmentof optical members. Further, it is a second object of the presentinvention to provide a high-precision position detecting apparatusutilizing the heterodyne interference method, solving the second andthird problems as described, which can avoid the negative influence ofthin film interference of resist and the negative influence of asymmetryof alignment mark.

As described above, it is necessary in the heterodyne interferencemethod that when two beams are converged at a single point a frequencydifference between two beams at a time is a predetermined value. Forthis, a double-beam light source apparatus of the present invention isso arranged that a single beam is emitted from a source and thatseparation into two beams or provision of frequency difference betweenthe two beams is carried out at a single point in a single elementassuring coincidence of optical path length with high accuracy.

Specifically, a double-beam light source apparatus of the presentinvention comprises (a) a single-beam light source for supplying asingle collimated beam and (b) frequency difference producing means forreceiving the beam emitted from the light source means, splitting itinto two beams while producing a predetermined frequency differencebetween the two beams with the two beams radially spreading.

The single-beam light source emits a collimated beam toward a beamsplitter. The emitted beam may be light of single wavelength or light ofmultiple wavelengths such as white light.

A frequency difference producer of a first type used in the double-beamlight source apparatus of the present invention comprises (i) a beamsplitter receiving a beam emitted from the single-beam light source andsplitting it into two beams traveling in mutually different directions,(ii) a first relay optical system for receiving and condensing the twobeams leaving the beam splitter, and (iii) a modulator disposed at aposition where the two beams are converged by the first relay opticalsystem, for changing a frequency in accordance with an incidentdirection and for changing an outgoing direction.

The beam splitter splits the incident beam into two beams having anidentical wavelength and traveling in the different directions withwavefronts thereof being aligned. The beam splitter is preferably adiffraction grating. Employing a diffraction grating, two beams havingthe same wavelength may be outgoing to travel in different directionswith wavefronts being aligned by selecting light of n-th order and -n-thorder with wavelength satisfying the diffraction condition.

The two split beams are condensed inside the modulator through the firstrelay optical system. The modulator may be one of (i) acousto-opticmodulators, (ii) radial grating devices, (iii) electro-optic modulators, (iv) photochromic devices, and (v) liquid crystal devices, in which adiffraction grating periodically changing is produced by acoustic means,rotational motion means, or electrical means. The two beams aresubjected to Bragg diffraction in the thus produced diffraction gratingto produce diffracted light. Properly selecting components or beams fromthe diffracted light, two beams traveling in different directions andhaving aligned wavefronts as well as a predetermined frequencydifference can be obtained.

If the formation direction of diffraction grating in changer (forexample, a traveling direction of acoustic signal (compressional wave)in case of acousto-optic modulator) is arranged to intersect with aplane including vectors or traveling directions of the two incidentbeams, mixture of zeroth order light into diffracted light can beavoided so as to reduce noise components in the two beams obtained.

A frequency difference producer of a second type used in the double-beamlight source apparatus of the present invention is constructed byproviding a second modulator behind the first type frequency differenceproducer. In detail, the second type frequency difference producercomprises (i) a beam splitter for receiving a beam emitted from asingle-beam light source to split it into two beams traveling inmutually different directions, (ii) a first relay optical system forreceiving and condensing the two beams outgoing from the beam splitter(iii), a first modulator provided at a position where the two beams areconverged by the first relay optical system, for modulating a frequencyin accordance with an incident direction and for changing an outgoingdirection, (iv) a second relay optical system for receiving andcondensing the two beams outgoing from the first changer, and (v) asecond modulator provided at a position where the two beams areconverged by the second relay optical system, for modulating a frequencyin accordance with an incident direction and for changing an outgoingdirection.

The second type frequency difference producer is so arranged that a beamfrom the single beam light source is guided through the beam splitter,the first relay optical system and the first modulator to obtain the twobeams traveling in mutually different directions and having and apredetermined frequency difference with wavefronts thereof beingmutually aligned, similarly as in the first type frequency differenceproducer. The two beams are converged inside the second modulatorthrough the second relay optical system. The second modulator may be oneselected from (i) acousto-optic modulators, (ii) radial grating devices,(iii) electro-optic modulators, (iv) photochromic devices, and (v)liquid crystal devices, as in the first type modulator, in which aperiodically changing diffraction grating is produced by acoustic means,rotational motion means, or electrical means. In this arrangement, theformation direction of diffraction grating of first modulator and theformation direction of diffraction grating of second modulator may bemade opposite to each other with respect to the traveling direction ofthe beams with a pitch of each diffraction grating being adjusted. Thefrequency difference between the two beams finally obtained may belowered to a degree easy in processing, if the frequency differenceobtainable with a single changer is too big. It is preferable inadjustment of frequency difference that the first modulator and thesecond modulator are of the same type. Properly selecting diffractedlight components from beams Bragg-diffracted by the thus produceddiffraction grating, one can obtain two beams traveling in differentdirections and having aligned wavefronts and a frequency differencewithin a range easy in signal processing.

If the formation directions of diffraction gratings in the first andsecond modulators are arranged to intersect with a plane includingtraveling-direction vectors of the two incident beams, the mixture ofzeroth order light into diffracted light can be effectively prevented toreduce noise components in the two beams.

A frequency difference producer of a third type used in the double-beamlight source apparatus of the present invention comprises asplitter-modulator composed of a single element serving as the beamsplitter and the modulator in the first type frequency differenceproducer, omitting the first relay optical system. Thesplitter-modulator includes a single acousto-optic modulator, in which asingle beam incident thereinto is subjected to the Raman-Nathdiffraction by a diffraction grating pattern formed by compressionalwave applied to the acousto-optic modulator, thereby to obtain two beamstraveling in two predetermined directions and having aligned wavefrontsand a predetermined frequency difference.

A frequency difference producer of a fourth type used in the double-beamlight source apparatus of the present invention is obtained by arranginga further modulator behind the third type frequency difference producer.In more detail, the fourth type frequency difference producer comprises(i) a splitter-modulator for receiving a beam emitted from the singlebeam light source to split it into two beams traveling in mutuallydifferent directions and for applying a predetermined change offrequency to the two beams, (ii) a relay optical system for receivingand condensing the two beams outgoing from the beam split-frequencymodulator, and (iii) a modulator provided at a position where the twobeams are converged by the relay optical system, for modulating afrequency in accordance with an incident direction and for changing anoutgoing direction.

The fourth type frequency difference producer is so arranged that thesplitter-modulator splits and modulates the single beam from the singlebeam light source into two beams traveling indifferent directions andhaving aligned wavefronts and a predetermined frequency difference, inthe same manner as in the third type frequency difference producer. Thetwo beams are converged inside the modulator through the relay opticalsystem. The modulator is an acoustic-optic modulator, in which aperiodically changing diffraction grating is produced by application ofacoustic signals. By setting the formation direction of diffractiongrating of the splitter-modulator and the formation direction ofdiffraction grating of the modulator opposite to each other with respectto the traveling direction of the beams and by adjusting the pitch ofthe diffraction gratings, one can lower a frequency difference betweenthe two beams finally obtained to a degree easy in processing, if thefrequency difference obtained by the splitter-modulator is too big. Thebeams are subjected to the Raman-Nath diffraction by the thusproduced-diffraction grating in the splitter-modulator and to the Braggdiffraction by the diffraction grating in the modulator. Properlyselecting diffracted light components, one can obtain two beamstraveling in different directions and having aligned wavefronts and afrequency difference within a range easy in signal processing.

If the formation direction of diffraction grating in modulator isarranged to intersect with a plane including traveling-direction vectorsof two incident beams, the mixture of zeroth order light into thediffracted light may be effectively prevented to reduce noise componentsin the two beams.

A position detecting apparatus of the present invention uses one of thedouble-beam light source apparatus of the present invention asdescribed, which detects a relative position between objects to bemeasured by the heterodyne interference method. In more detail, theposition detecting apparatus of the present invention comprises (i) thedouble-beam light source apparatus of the present invention, (ii) a beamseparator for separating two beams outgoing from the double-beam lightsource apparatus into two sets of double beams, (iii) a first condenserfor condensing a set of double beams outgoing from the beam separator,(iv) a reference diffraction grating provided at the condensing positionof the first condenser, (v) a first photodetector for detecting anoptical image of diffracted light from the reference diffractiongrating, (vi) a second condenser for condensing the other set of doublebeams outgoing from the beam separator, (vii) a second photodetector fordetecting an optical image of diffracted light from a diffractiongrating on an object to be measured, disposed at a condensing positionof the second condenser, and (viii) an optical image processor forreceiving an output signal of the first photodetector and an outputsignal of the second photodetector to compare the both optical imageswith each other and for calculating a displacement amount of themeasured object from the reference position.

Further, in addition to the arrangement of the above position detectingapparatus, an apparatus may comprise a third condenser for againcondensing the two beams diffusing after condensed by the secondcondenser, and a third photodetector for detecting an optical image ofdiffracted light by a diffraction grating on a second object to bemeasured, disposed at a condensing position of the third condenser, inwhich the optical image processor receives an output signal of the thirdphotodetector in addition to the output signals of the first and secondphotodetectors and a relative positional deviation is obtained betweenthe position of the first measured object and the position of the secondmeasured object, whereby the deviation may be accurately measuredbetween the two objects to be measured. The apparatus may furthercomprise a driver for the first measured object and a driver for secondmeasured object, in which a deviation between the measured objects isadjusted for alignment, whereby a precise aligning apparatus may beattained.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art form this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing to show the schematic structure of the firstembodiment according to the present invention;

FIG. 2 and FIG. 3 are plan views to show appearance of diffractiongrating marks;

FIG. 4 and FIG. 5 are plan views to show appearance of field aperturesprovided in alignment optical system;

FIG. 6 is a drawing to show the structure of a double-beam producingportion (which is a portion producing two beams different in frequencyfrom each other) in the first embodiment;

FIG. 7 is a drawing to illustrate the theory of acoustic Braggdiffraction by acousto-optic modulator;

FIG. 8 and FIG. 9 are drawings to show a situation to produce noiselight in acousto-optic modulator in the first embodiment;

FIG. 10 is a drawing to show the schematic structure of the secondembodiment according to the present invention;

FIG. 11 is a drawing to show the structure of a double-beam producingportion (which is a portion producing two beams different in frequencyfrom each other) in the second embodiment;

FIGS. 12-15 are drawings to show a situation to produce noise light inthe second acousto-optic modulator in the second embodiment;

FIG. 16 is a schematic constitutional drawing to show a projectionexposure apparatus to which the first embodiment of position detectingapparatus according to the present invention is applied;

FIG. 17 is a drawing to illustrate the operation of acousto-opticmodulator 17a in the third embodiment;

FIG. 18 is an explanatory drawing of the theory of Raman-Nathdiffraction by acousto-optic modulator;

FIG. 19 is a schematic constitutional drawing to show a projectionexposure apparatus in the fourth embodiment of the present invention;

FIG. 20 is a drawing to illustrate the operation of two acousto-opticmodulators 17a and 60 in the fourth embodiment.

FIG. 21 is a perspective view to show an arrangement of optical membersof from diffraction grating 14 to space filter 19 in the fifthembodiment;

FIG. 22 is a perspective view to show an arrangement of optical membersof from diffraction grating 14 to space filter 19;

FIG. 23 is a perspective view to show an arrangement of optical membersof from relay lens 18b to space filter 61;

FIG. 24 is a drawing to show an example in which light source means iscomposed of a plurality of laser sources emitting beams different inwavelength from each other and a blazed diffraction grating;

FIG. 25 is a drawing to show an example in which beam splitting means iscomprised of a Wollaston prism;

FIG. 26 is an optical path diagram to illustrate another productionmethod of two beams with diffraction grating 14;

FIG. 27 is a schematic constitutional drawing to show a projectionexposure apparatus in a modification of the present invention;

FIG. 28 is a drawing to illustrate the operation of two acousto-opticmodulators 70, 17 in the modification;

FIG. 29 is a perspective view to show an arrangement of optical membersof from AOM 70 to space filter 19;

FIG. 30 is an explanatory view to illustrate a radial grating; and

FIG. 31 is an explanatory drawing to illustrate an example of formationof diffraction grating by voltage application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be described indetail with reference to the accompanying drawings. Projection exposureapparatus will be described in the preferred embodiments, in which thedouble-beam light source apparatus and the position detecting apparatusaccording to the present invention are employed.

First Embodiment

FIG. 1 is a schematic constitutional drawing of the first embodiment toshow a projection exposure apparatus in which the double-beam lightsource and the position detecting apparatus of the present invention areused. The double-beam light source in the first embodiment employs thefirst type frequency difference producer as described before. The firstembodiment will be described in detail with reference to FIG. 1.

A reticle (mask) 1 has a predetermined circuit pattern and a diffractiongrating mark RM for alignment disposed at the peripheral portion ofpattern and is held on a reticle stage 2, which is two-dimensionallymovable. The reticle 1 is positioned conjugate with a wafer (substrate)4 with respect to a projection objective lens 3.

Exposure light from an illumination optical system 40 is reflecteddownward by a dichroic mirror 6 inclined at 45° above the reticle touniformly illuminate the reticle 1. The pattern on the illuminatedreticle is replicated onto the wafer 4 through the projection objectivelens 3. Formed on the wafer 4 is a diffraction grating mark WM foralignment, which is similar to the diffraction grating mark RM formed onthe reticle.

The wafer 4 is held on a wafer stage 5, which two-dimensionally moves instep-and-repeat process. The wafer is stepped to a next shot positionafter completion of replication of reticle pattern at a shot area.

There are unrepresented interferometers, one for each of the reticlestage 2 and the wafer stage 5, to independently detect positions of thestages in the x direction, in the y direction and in the rotational (θ)direction. Each stage is driven by an unrepresented drive motor in thedirections.

An alignment optical system for position detection is disposed above thedichroic mirror 6. The alignment optical system is described in detailin the following.

A white light source 10 such as a Xe lamp and a halogen lamp emits whitelight in wavelength band different from that of the exposure light. Thewhite light is guided through a variable aperture diaphragm 11 and acondenser lens 12 to be converted into a collimated beam L₀. Thecollimated beam L₀ is guided through a band-pass filter 13 extractinglight in predetermined wavelength range to vertically illuminate adiffraction grating 14. The collimated beam L₀ vertically illuminatingthe diffraction grating 14 is split into ±1st order diffracted lightcomponents or beams (L₁, L₂) having the predetermined wavelength rangeby the diffraction effect of the diffraction grating 14.

The ±1st order diffracted light components (L₁, L₂) having thepredetermined wavelength range are converged by a relay optical systemhaving lenses 15a and 15b. Then, the condensed beams enter anacousto-optic modulator (as will be referred to as AOM) 17 at the sameincident angle in symmetry with each other. A space filter 16 isdisposed between the lenses 15a and 15b to extract the ±1st orderdiffracted light components leaving the diffraction grating 14.

The AOM 17 is driven with a high frequency signal SF₁ of frequency f₁.The beams L₁ and L₂ in predetermined wavelength range are subjected tothe acoustic Bragg diffraction in AOM 17.

Suppose the beams L₁ and L₂ in predetermined wavelength rangerespectively have a frequency f₀. Then, +1st order diffracted light L₁(1) (as will be referred to as beam L₁ (1)) of the beam L₁ inpredetermined wavelength range is frequency-modulated by the AOM 17 tohave a frequency of f₀ +f₁, while -1st order diffracted light L₂ (-1)(as will be referred to as beam L₂ (-1)) of the beam L₂ in predeterminedwavelength range is frequency-modulated by the AOM 17 to have afrequency of f₀ -f₁.

After that, the beam L₁ (1) and the beam L₂ (-1) pass through a lens18a, a reflection mirror 20, a lens 18b and a lens 21 and then is splitinto two by a beam splitter 22. A space filter 19 is provided betweenthe lens 18 and the lens 18b constituting a relay optical system toextract the +1st order diffracted light beam L₁ (1) and the -1st orderdiffracted light beam L₂ (-1).

The beam L₁ (1) and the beam L₂ (-1) passing through the beam splitter22 are converged by a lens 23 and form interference fringes flowingalong the pitch direction on a diffraction grating 24 for referencedisposed at the condensing position of the lens 23. Diffracted lightthrough the diffraction grating 24 is photoelectrically detected asoptical beat signal for reference by a detector 25.

On the other hand, the beam L₁ (1) and the beam L₂ (-1) reflected by thebeam splitter 22 pass through a relay optical system (26a, 26b, 27), abeam splitter 28 and a parallel-plane plate 37.

The parallel-plane plate 37 is located at or near the pupil-conjugateposition of the projection objective lens such that its inclinationangle is variable to the optical axis of alignment optical system,having a function to maintain the system telecentric. The parallel-planeplate may be replaced by a combination of a thick parallel-plane platefor coarse adjustment with a thin parallel-plane plate for fineadjustment.

The beam L₁ (1) and the beam L₂ (-1) passing through the parallel-planeplate 37 then passes through an objective lens 38 and the dichroicmirror 6 to illuminate the diffraction grating mark RM on the reticle 1in two directions having a predetermined cross angle.

In case that the projection lens 3 is not achromatic to the alignmentlight, the objective lens 38 is preferably a bifocal optical system asproposed in Japanese Laid-open Patent Application No. 63-283129. In thisarrangement, two beams entering the bifocal optical system are splitrespectively into polarized light components perpendicular to eachother. One of the two polarized light components from each beam directedtoward the first focus is converged on the reticle, while the other ofthe two polarized light components from each beam directed toward thesecond focus is converged on the wafer.

As the beam L₁ (1) and the beam L₂ (-1) illuminate the diffractiongrating mark RM on the reticle, the reticle 1 has an alignment lighttransmitting window P₀ next to the diffraction grating mark RM, as shownin FIG. 2, and the diffraction grating mark WM is formed at the positioncorresponding to the transmitting window P₀ on the wafer, as shown inFIG. 3.

The beams L₁ (1) and L₂ (-1) illuminate the diffraction grating mark RMand the transmitting window P₀ as covering them, so that interferencefringes flowing along the pitch direction are formed on the diffractiongrating mark RM. +1st order diffracted light of the beam L₁ (1) and -1storder diffracted light of the beam L₂ (-1) advance in the directionnormal to the diffraction grating mark RM (along the optical axis ofprojection objective lens 3).

The beams L₁ (1) and L₂ (-1) are arranged to illuminate the diffractiongrating mark RM in the two directions with a cross angle satisfying thefollowing relation.

    sin θ.sub.RM =λ.sub.0 /P.sub.RM               (1)

where P_(RM) is a pitch of diffraction grating mark RM, λ₀ a basewavelength of light emitted from the light source 10, and θ_(RM) anincident angle of beam L₁ (1) or beam L₂ (-1) into the diffractiongrating mark RM.

By this, ±1st order diffracted light beams from the diffraction gratingmark RM are guided to pass again through the dichroic mirror 6, theobjective lens 38 and the parallel-plane plate 37. After that, the ±1storder diffracted light beams are reflected by the beam splitter 28 topass through a lens 29 and a beam splitter 30 then to reach a fieldaperture 34.

The field aperture 34 is located conjugate with the reticle 1.Specifically, the field aperture 34 has an opening portion S_(RM) at theposition corresponding to the diffraction grating mark RM to permit onlydiffracted light from the diffraction grating mark RM on the reticle 1to pass therethrough, as shown in FIG. 4.

Then, the diffracted light from the diffraction grating mark RM passingthrough the field aperture 34 is filtered by a space filter 35 forcutting zeroth order diffracted light, so that only ±1st orderdiffracted light reaches a detector 36. The photoelectric detector 36photoelectrically detects an optical beat signal including informationon position of reticle 1.

A part of beams L₁ (1) and L₂ (-1) passing through the transmittingwindow P₀ of reticle 1 illuminate through the projection objective lens3 the diffraction grating mark WM on the wafer 4 in two directions witha certain cross angle, so that interference fringes flowing along thepitch direction are formed on the diffraction grating mark WM. -1storder diffracted light of the beam L_(l) (1) and +1st order diffractedlight of the beam L₂ (-1) are made to advance in the direction normal tothe diffraction grating mark WM (along the optical axis of projectionobjective lens 3).

The beams L₁ (1) and L₂ (-1) are arranged to illuminate the diffractiongrating mark RM in two directions with a cross angle satisfying thefollowing relation.

    sin θ.sub.WM =λ.sub.0 /P.sub.WM,              (2)

where P_(WM) is a pitch of diffraction grating mark WM, λ₀ a referencewavelength of light emitted from light source 10, and θ_(WM) an incidentangle of beam L₁ (1) or beam L₂ (-1) into the diffraction grating markWM.

Then, ±1st order diffracted light from diffraction grating mark WM thenpass again through the projection objective lens 3, the transmittingwindow P₀, the dichroic mirror 6, the objective lens 38 and theparallel-plane plate 37, and is reflected by the beam splitter 28 topass through the lens 29 and the beam splitter 30 then to reach a fieldaperture 31.

The field aperture 31 is located conjugate with the wafer 4.Specifically, the field aperture 31 has an opening portion S_(WM) at theposition corresponding to the diffraction grating mark WM to permit onlydiffracted light from the diffraction grating mark WM on the wafer 4 topass therethrough, as shown in FIG. 5.

The diffracted light from the diffraction grating mark WM passingthrough the field aperture 31 is filtered by a space filter 32 forcutting zeroth order diffracted light, so that only ±1st orderdiffracted light beams reach a detector 33. The photoelectric detector33 photoelectrically detects an optical beat signal includinginformation on position of wafer 4.

Each space filter (32, 35) is located at a position approximatelyconjugate with the pupil of alignment optical system, that is,substantially conjugate with the pupil (exit pupil) of projectionobjective lens 3 so that the zeroth order diffracted light (regularreflection light) from diffraction grating mark (RM, WM) formed onreticle 1 or wafer 4 is interrupted and that only ±1st order diffractedlight (diffracted light advancing normal to the diffraction grating markon reticle 1 or wafer 4) can pass therethrough. Also, each detector (33,36) is disposed approximately conjugate with reticle 1 or wafer 4 withrespect to the objective lens 38 and the lens 29.

In the arrangement of alignment optical system as described above, eachof three signals obtained by the detectors (25, 33, 36) includes asinusoidal optical beat signal of same frequency Δf (=|2f₁ |). Anoptical beat signal extracting portion (Fourier transform circuit) inphase difference detection system 50 effects the Fourier transformelectrically on the three photoelectric signals, whereby threesinusoidal optical beat signals of frequency Δf are extracted atexcellent precision.

Suppose the reticle 1 and the wafer 4 are stopped at arbitraryrespective positions before aligned. Then, the optical beat signals willhave a certain phase difference. A phase difference of ±180° between theoptical beat signals from reticle 1 and from wafer 4 is uniquelycorrespondent to a displace amount of relative position within a half ofthe grating pitch of diffraction grating marks respectively formed onthe reticle 1 and on the wafer 4.

Thus, as the reticle 1 is moved relative to the wafer 4 in the directionof grating arrangement, pre-alignment is carried out at precision ofrelative position deviation amount below a half of the grating pitch ofdiffraction grating marks (RM, WM) and then a main control system 51performs alignment by two-dimensionally moving the reticle stage 2 orthe-wafer stage 5 through a servo system 52 as to make a phasedifference obtained by the phase difference detection system 50 equal tozero or a certain value, whereby high-resolution position detection maybe achieved.

Alternatively, using the optical beat signal for reference obtained bythe detector 25 as a reference signal, the alignment may be conducted asto make a phase difference equal to zero or a certain value between thereference signal and the optical beat signals from diffraction gratingmarks (RM, WM). Also, a drive signal for driving the AOM 17 may be usedas a reference signal.

Next described with FIG. 6 are a specific construction and the theory inproduction of two beams different in frequency from each other in thefirst embodiment shown in FIG. 1.

When white light L₀ illuminates the diffraction grating 14 in thedirection normal thereto, as shown in FIG. 6, diffracted light appearsby diffraction through the diffraction grating 14, including componentsof various orders for each wavelength.

The diffracted light includes components of various orders satisfyingthe following condition (3):

    sin Φ.sub.1 =Nλ/P.sub.G                         (3)

where Φ₁ is a diffraction angle of diffracted light with respect to thedirection of normal line to the diffraction grating 14, P_(G) pitch ofdiffraction grating 14, λ a wavelength of light, and N (integers) theorder of diffracted light component.

Then, the diffracted light including the various order components in acertain wavelength band passes through the condensing lens 15a and isfiltered by the space filter 16 disposed at the rear focus position ofcondenser lens 15a (or at the front focus position of condenser lens15b) so that components other than ±1st order diffracted lightcomponents in predetermined wavelength band are shielded and that onlythe ±1st order diffracted light components (L₁, L₂) in predeterminedwavelength band pass through the filter 16 then to go through thecondenser lens 15b toward the AOM 17.

Let us now consider the diffraction angle of ±1st order diffracted lightcomponents (L₁, L₂) passing through the space filter 16. For example,suppose the base wavelength λ₀ of irradiation light L₀ is 633 nm, thewavelength band of irradiation light L₀ is λ₀ ±Δλ (=20 nm), and thepitch of diffraction grating 14 is 4 μm. Then, the diffraction angle of±1st order diffracted light components is 8.82° for minimum wavelengthof 613 nm, while 9.40° for maximum wavelength of 653 nm by aboveEquation (3).

Accordingly, light having the wavelength band of 613 nm-653 nm supplies±1st order diffracted light components with diffraction angle in a rangeof 8.82°-9.40°.

As described, the diffraction angle Φ₁ varies depending upon thewavelength in light. In the present embodiment, as shown in FIG. 6, therelay optical system 16 relays the diffraction point of diffractiongrating 14 into the traveling path of ultrasonic wave in AOM 17 so that±1st order diffraction light components of each wavelength are convergedinside the AOM 17. Therefore, the ±1st order diffracted light components(L₁, L₂) in predetermined wavelength band, which are symmetrically splitinto two by the diffraction grating 14, enter the AOM 17 symmetricallyat an incident angle Φ₂ preset for each wavelength.

This will be described in more detail using equations. First, the ±1storder diffracted light components (L₁, L₂) in predetermined wavelengthrange enter the AOM 17 at the incident angle Φ₂ in two directions andare subject to the acoustic Bragg diffraction in AOM 17.

The AOM 17 is driven by a high-frequency signal SF₁ of such a frequencyf₁ that the +1st order diffracted light L₁ in predetermined wavelengthrange produces +1st order diffracted light L₁ (1) with diffraction angleof 2Φ₂ (double of incident angle Φ₂) and that the -1st order diffractedlight L₂ in predetermined wavelength range produces -1st orderdiffracted light L₂ (-1) with diffraction angle of 2Φ₂.

Letting the diffraction angle by Bragg diffraction of AOM 17 be θ_(b1)(=2Φ₂), a velocity of ultrasonic wave (traveling wave) crossing the AOM17 be v₁, the frequency of ultrasonic wave of high-frequency signal SF₁be f₁, the wavelength of light be λ, and the wavelength of ultrasonicwave (traveling wave) crossing the AOM 17 be Λ₁, the following relationsof Equations (4) and (5) stand.

    Λ.sub.1 =v.sub.1 /f.sub.1                           (4)

    sin θ.sub.b1 =λ/Λ.sub.1                (5)

From above Equations (4) and (5), the diffraction angle θ_(b1) (=2Φ₂) byAOM 17 is finally obtained by following Equation (6).

    sin θ.sub.b1 =f.sub.1 λ/v.sub.1 (or sin 2Φ.sub.2 =f.sub.1 λ/v.sub.1)                                         (6)

Therefore, the +1st order diffracted light L₁ (1) in predeterminedwavelength range and the -1st order diffracted light L₂ (-1) inpredetermined wavelength range symmetrically leave the AOM 17 at adiffraction angle for each wavelength satisfying above Equation (6).

Supposing the magnification of relay optical system (15a, 15b) is β₁ andthe relay optical system (15a, 15b) satisfies the sine condition, thefollowing relation stands.

    β.sub.1 =sin Φ.sub.1 /sin Φ.sub.2 ≈2 sin Φ.sub.1 /sin (2Φ.sub.2)                                       (7)

The following Equation (8) can be derived from Equations (3), (6) and(7).

    β.sub.1 =(2v.sub.1)/(P.sub.G f.sub.1)                 (8)

The relay optical system (15a, 15b) is preferably arranged to satisfyabove Equation (8) accordingly.

Next described with FIG. 7 is the light frequency modulation by theacoustic Bragg diffraction. In FIG. 7, θ_(i) represents an angle betweenthe wavefront of ultrasonic wave of drive signal SF₁ in AOM 17 and theincident light (incident angle of incident light), θ_(d) an anglebetween the wavefront of ultrasonic wave of drive signal SF₁ in AOM 17and diffracted light, K_(i) a wave vector of incident light into AOM 17,K_(d) a wave vector of diffracted light by AOM 17, and K_(s) a wavevector of ultrasonic wave of high-frequency signal SF₁.

If incident light and diffracted light satisfy the condition of acousticBragg diffraction, the vectors are in relation of isosceles triangle.With wavelength λ of light, index of refraction n of AOM 17, frequencyf₁ of ultrasonic wave, and velocity v₁ of ultrasonic wave (travelingwave) crossing the AOM 17, the amplitudes of K_(i), K_(d) and K_(s) maybe expressed as follows.

    |K.sub.i |=2πn/λ               (9)

    |K.sub.d |=2πn/λ               (10)

    |K.sub.s |=2πf.sub.1 /v.sub.1         (11)

Further, θ_(i) and θ_(d) are equal to each other, and thus are rewrittenas θ₀. Letting the wavelength of ultrasonic wave be Λ₁, the followingrelations stand.

    sin 2θ.sub.0 =λ/Λ.sub.1                (12)

    |K.sub.s |=2(sin θ.sub.0)·|K.sub.d |      (13)

Assuming sin 2θ₀ ≈2(sinθ₀), following Equation (14) may be derived fromEquations (12) and (13).

    |K.sub.s |=2πn/Λ.sub.1         (14)

As apparent from this Equation (14), the amplitude |K_(s) | is constantirrespective of wavelength of light as far as the Bragg diffractioncondition is satisfied. It is, therefore, understood that lightdiffracted by AOM 17 is subject to the same frequency modulation (f₁)regardless of wavelength of light.

If the beams (L₁, L₂) incident into AOM 17 in two directions have afrequency f, +1st order diffracted light L₁ (1) of the beam L₁ isequally frequency-modulated to have a frequency of f+f₁ (=F₁); -1storder diffracted light L₂ (-1) of the beam L₂ is equallyfrequency-modulated to have a frequency of f-f₁ (=F₂) .

Since the +1st order diffracted light L₁ (1) of frequency F₁ inpredetermined wavelength range and the -1st order diffracted light L₂(-1) of frequency F₂ in predetermined wavelength range can symmetricallyilluminate the diffraction gratings (24, RM, WM) with a component ofeach wavelength impinging at an incident angle different from others,±1st order diffracted light components of each wavelength can alwaysappear in the direction normal to the respective diffraction gratings(24, RM, WM). This results in production of beat light including apredetermined frequency (Δf=|F₁ -F₂ |=|2f₁ |) from the ±1st orderdiffracted light components Of each wavelength. Thus, the beat light ofmultiple wavelengths including the predetermined frequency (Δf=|F₁ -F₂|=|2f₁) can be photoelectrically detected by the respective detectors(25, 33, 36) (some wavelength components are detected from the beatlight including positional information of each diffraction grating).Therefore, high-precision alignment can be achieved in heterodyneinterference method while suppressing the influence of asymmetry of eachdiffraction grating mark by averaging effect of beat light signals ofsome wavelengths as well as the influence of thin film interference ofresist (influence such as a change in light quantity) with multiplewavelength light.

In addition, the white light (multi-wavelength light) is symmetricallysplit with respect to the incident direction (direction of optical axis)by the diffraction grating 14 (beam splitting means) and the thus splitlight travels symmetrically and concurrently through the relay opticalsystem and the AOM 17, whereby no difference of optical path length istheoretically present between the two split beams. Therefore, thewavefronts of the split beams are aligned with zero phase difference, sothat high-precision alignment becomes possible, achieving a compactapparatus easy in adjustment.

In the first embodiment, the ±1st order diffracted light components (L₁(1), L₂ (-1)) optically modulated in and symmetrically leaving the AOM17 are used as beams for alignment, the ±1st order diffracted lightcomponents (L₁ (1), L₂ (-1)) are guided to illuminate the diffractiongratings (24, RM, WM) in two directions, the diffraction gratingsproduce signals of beat light of predetermined frequency (Δf=|2f₁ |)upon the irradiation, and the signals of beat light are extractedthrough the detectors (25, 33, 36) and the optical beat signalextracting portion (Fourier transform circuits) in the phase differencedetection system 50 to utilize extracted signals for alignment. Itsreason is as follows.

In the present embodiment, as shown in FIG. 8 and FIG. 9, zeroth orderdiffracted light L₂ (0) of the beam L₂ is mixed in an optical path A offirst order diffracted light L₁ (1), and zeroth order diffracted lightL₁ (0) of the beam L₁ in optical path B of -1st order diffracted lightL₂ (-1) upon optical modulation in AOM 17. Upon optical modulation, thezeroth order diffracted light L₁ (0) of the beam L₁ and the zeroth orderdiffracted light L₂ (0) of the beam L₂ are free of frequency modulationfor each wavelength, maintaining the original frequency f₀.

Thus, there are mixed in optical path A the first order diffracted lightL₁ (1) of frequency of f₀ +f₁ and the zeroth order diffracted light L₂(0) of frequency of f₀. Also in optical path B, there are mixed the -1storder diffracted light L₂ (-1) of frequency of f₀ -f₁ and the zerothorder diffracted light L₁ (0) of frequency of f₀. If they are guided toilluminate the diffraction gratings (24, RM, WM) in two directions, beatlight is produced with various beat frequencies in the direction normalto the respective diffraction gratings (24, RM, WM). Then, if thealignment is carried out based on signals obtained by simplyphotoelectrically converting the beat light having various beatfrequencies by means of the detectors (25, 33, 36), signals of variousbeat frequencies would be noise signals, negatively affecting thedetection precision and making the alignment impossible.

First, study is made in the following on the beat light having variousbeat frequencies produced by optical modulation of AOM 17.

There are frequencies of diffracted light components listed below.

In optical path A:

frequency of +1st order diffracted light L₁ (1): f₀ +f₁ (I)

frequency of zeroth order diffracted light L₂ (0): f₀ (II).

In optical path B:

frequency of -1st order diffracted light L₂ (-1): f₀ -f₁ (I')

frequency of zeroth order diffracted light L₁ (0): f₀ (II').

Thus, the beat light may have the following frequencies from thecombination of diffracted light traveling in optical path A withdiffracted light traveling in optical path B.

Taking an absolute value of difference between (I) and (I'),

    |(f.sub.0 +f.sub.1)-(f.sub.0 -f.sub.1)|=|2f.sub.1 |         [1].

Taking an absolute value of difference between (I) and (II'),

    |(f.sub.0 +f.sub.1)-f.sub.0 |=|f.sub.1 |[2].

Taking an absolute value of difference between (II) and (I'),

    |f.sub.0 -(f.sub.0 -f.sub.1)|=|f.sub.1 |[3].

Taking an absolute value of difference between (II) and (II'),

    |f.sub.0 -f.sub.0 |=0                    [4].

Therefore, the beat light photoelectrically detected by the detectors(25, 33, 36) includes three beat frequencies of [1]-[3]. If thecomponent of [4] is included in the diffracted light andphotoelectrically detected by the detectors (25, 33, 36), it will be adirect current component (DC component). In case that the DC componentnegatively affects the detection precision, it may be removed when theFourier transform is effected in the optical beat signal extractingportion in phase difference detection system 50. Alternatively, the DCcomponent may be removed by electrical filter means separately provided.A beat frequency which can be utilized for alignment is one which doesnot appear twice in the listed beat frequencies. Therefore, in thepresent embodiment a signal of beat frequency of |2f₁ |, which is onlyone beat light produced and appearing once in the combination of beam L₁(1) with beam L₂ (-1), is extracted by the optical beat signalextracting portion (Fourier transform circuit) in phase differencedetection system 50.

Even if the detectors (25, 33, 36) photoelectrically detect beat lighthaving various beat frequencies, high-precision alignment can beachieved by heterodyne interference, based on the signal ofpredetermined beat frequency (|2f₁ |) extracted by the optical beatsignal extracting portion (Fourier transform circuit) in the abovearrangement.

By properly arranging the diffraction grating 14 and the AOM 17,unnecessary diffracted light L₂ (0) traveling in optical path A of beamL₁ (1) and unnecessary diffracted light L₁ (0) traveling in optical pathB of beam L₂ (-1) can be separated out and filtered by the space filter19, as described hereinafter.

Although the first embodiment shown in FIG. 1, FIG. 6 and FIGS. 8 and 9shows an example in which the ±1st order diffracted light beams split bythe diffraction effect of diffraction grating 14 are guided to enter theAOM 17 as two beams for position detection and in which +1st order lightfrom one of the two beams diffracted through AOM 17 and -1st order lightfrom the other of the two beams diffracted through AOM 17 are guided toirradiate marks for position detection in two directions as two beamsfor position detection, the invention is not limited to this example.For example, any two diffracted light components of arbitrary orderproduced by the diffraction grating 14 can be used as two beams forposition detection as guided to enter the AOM 17. Further, anotherarrangement is such that a diffracted light component of arbitrary orderfrom one of two beams diffracted through AOM 17 and a diffracted lightcomponent of arbitrary order from the other of two beams diffractedthrough AOM 17 are guided to illuminate marks for position detection intwo directions.

Although the first embodiment shown in FIG. 1 shows an example in whichthe light source means (10-12) supplies a beam including light of pluralwavelengths (multiple wavelengths), the invention is not limited to thisexample. The light source means may be a laser source supplying light ofsingle wavelength, whereby the alignment can be achieved in heterodyneinterference method. In this case, the advantage in the heterodyneinterference method using light of multiple wavelengths cannot beenjoyed in alignment, but two beams different in frequency from eachother can be produced without difference of optical path length bysimply arranging in series a diffraction grating and an acousto-opticmodulator on either side of a relay optical system. Thus, the apparatusso arranged is simple in structure and remarkably easy in adjustment ascompared with the conventional apparatus.

Second Embodiment

The second embodiment according to the present invention will bedescribed with reference to FIG. 10. The present embodiment employs thesecond type frequency difference producer as described before. In FIG.10 members having the same functions as those in the first embodimentshown in FIG. 1 are given the same reference numerals.

The present embodiment is different from the first embodiment in that asecond acousto-optic modulator 60 (as will be referred to as AOM 60) isprovided between the relay optical system (18a, 18b) and the condenserlens 21, that a space filter 61 is provided between the condenser lens21 and the beam splitter 22, and that the lens 18a and the lens 18bserve as a second relay optical system for relaying the diffractionpoint of AOM 17 (first acousto-optic modulator) to the diffraction pointof AOM 60 (second acousto-optic modulator).

In the present embodiment a high-frequency signal SF₂ applied to AOM 60travels in the direction opposite to the traveling direction of thehigh-frequency signal SF₁ applied to AOM 17, whereby a finally obtainedbeat frequency may be lowered (below 1 MHz) to facilitate the processingof electric signal.

As shown in FIG. 10, white light from white light source 10, whichsupplies light in a certain wavelength band (multiple wavelengths)different from that of exposure light, is guided to pass through avariable aperture 11, a condenser lens 12 and a band-pass filter 13 thento illuminate a diffraction grating 14 in the direction normal theretoand then is split by diffraction effect of the diffraction grating 14into ±1st order diffracted light (L₁, L₂) having a predeterminedwavelength range. After the ±1st order diffracted light (L₁, L₂) havingthe predetermined wavelength range is condensed by a relay opticalsystem (15a, 15b), it enters the AOM 17 symmetrically at equal incidentangle. A space filter 16 disposed between the lenses 15a and 15bextracts the ±1st order diffracted light leaving the diffraction grating14.

The AOM 17 is driven by a first high-frequency signal SF₁ of frequencyf₁. Assuming the beams L₁ and L₂ in predetermined wavelength rangerespectively have a frequency f, the beam L₁ in predetermined wavelengthrange is frequency-modulated by AOM 17 to produce ±1st order diffractedlight L₁ (1) (as will be referred to as beam L₁ (1)) of frequency (f₀+f₁), while the beam L₂ in predetermined wavelength range isfrequency-modulated by AOM 17 to produce -1st order diffracted light L₂(-1) (as will be referred to as beam L₂ (-1)) of frequency (f₀ -f₁).After that, the beams L₁ (1) and L₂ (-1) pass through a lens 18a, areflection mirror 20 and a lens 18b to symmetrically enter the AOM 60 atequal incident angle. A space filter 19 disposed in the second relayoptical system (15a, 15b) extracts only ±1st order diffracted light (L₁(1), L₂ (-1)) leaving the AOM 17.

The AOM 60 is driven by a second high-frequency signal of frequency f₂traveling in the direction opposite to that of AOM 17. The beam L₁ (1)in predetermined wavelength range is frequency-modulated by AOM 60 toproduce -1st order diffracted light L₁ (1, -1) (as will be referred toas beam L₁ (1, -1)) of frequency of f₀ +f₁ -f₂ (=F₁), while the beam L₂(-1) in predetermined wavelength range is frequency-modulated by AOM 60to produce +1st order diffracted light L₂ (-1, 1) (as will be referredto as beam L₂ (-1, 1)) of frequency of f₀ -f₁ +f₂ (=F₂). Then, the beamsL₁ (1, -1) and L₂ (-1, 1) pass through a lens 21 and are respectivelysplit into two by a beam splitter 22. The space filter 61 is provided inthe second relay optical system (18a, 18b) so that it extracts the -1storder diffracted light L₁ (1, -1) and +1st order diffracted light L₂(-1, 1) leaving the AOM 60.

The beams thus split into two by the beam splitter 22 arephotoelectrically detected by the detectors (25, 33, 36) at final stepin the same manner as in the first embodiment shown in FIG. 1 asdescribed before. Therefore, the details of the detection are omittedherein. In the present embodiment, an optical beat signal extractingportion (Fourier transform circuit) in phase difference detection system50 extracts beat signals of predetermined frequency (Δf=|F₁ -F₂ |=|2(f₁-f₂)|) formed by the light components of respective wavelengths, out ofphotoelectric signals photoelectrically detected by the detectors (25,33, 36), and the alignment is carried out based on these extractedsignals.

As described above, the present embodiment is so arranged that two AOMsare arranged in series and are driven by high-frequency signals (SF₁,SF₂) traveling in opposite directions, whereby the frequencies of beatsignals photoelectrically detected by the detectors (25, 33, 36) can belowered below 1 2 MHz, which is easy in signal processing.

For example, if the AOM 17 is driven by a first high-frequency signalSF₁ of frequency f₁ of 100 MHz and if the AOM 60 is driven by a secondhigh-frequency signal SF₂ of frequency f₂ of 99.9 MHz in the oppositedirection to that in AOM 17, after passing through AOM 60, the one beamL₁ (1, -1) has a frequency of F₁ =f₀ +f₁ -f₂ and the other beam L₂(-1, 1) a frequency of F₂ =f₀ -f₁ +f₂, as described above.

Then, the two beams (L₁ (1, -1), L₂ (-1, 1)) produce beat light offrequency of 200 KHz (Δf=|F₁ -F₂ |=|2(f₁ -f₂)|) through the diffractiongratings (24, RM, WM). Such beat frequency is easy in signal processing.

Next described with FIG. 11 is a more specific structure of the part forproducing two beams different in frequency from each other in the secondembodiment shown in FIG. 10.

As shown in FIG. 11, the second embodiment has such an arrangement thatthe diffraction grating 14, the first AOM 17 and the second AOM 60 aredisposed in series and further that there are provided the first relayoptical system (15a, 15b) for relaying the diffraction point of thediffraction grating 14 to the diffraction point of the first AOM 17 (intraveling path of high-frequency signal SF₁) and the second relayoptical system (18a, 18b) for relaying the diffraction point of thefirst AOM 17 (in traveling path of high-frequency signal SF₁) to thediffraction point of the second AOM 60 (in traveling path ofhigh-frequency signal SF₂).

The white light (multi-wavelength light) L₀ normally illuminating thediffraction grating 14 produces ±1st order diffracted light (L₁, L₂)symmetric with each other at an angle Φ₁ to the incident direction(direction of optical axis) by the diffraction effect of diffractiongrating 14. The ±1st order diffracted light (L₁, L₂) is condensed by thefirst relay optical system (15a, 15b) to symmetrically enter the firstAOM 17 at angle Φ₂ to the direction of optical axis. Since the first AOM17 is driven by a first high-frequency signal f₁, the beam L₁ isfrequency-modulated to produce the first order diffracted light L₁ (1)of frequency (f₀ +f₁) and the beam L₂ is frequency-modulated to producethe -1st order diffracted light L₂ (-1) of frequency (f₀ -f₁). The beams(L₁ (1), L₂ (-1)) leave the first AOM 17 symmetrically at angle Φ₂ equalto the incident angle Φ₂.

The beams L₁ (1) and L₂ (-1) optically-modulated by the first AOM 17 arecondensed by the second relay optical system (18a, 18b) and then enterthe second AOM 60 at angle Φ₃ to the direction of optical axis. Sincethe second AOM 60 is driven by a second high-frequency signal f₂ in theopposite direction to that of first AOM 17, the beam L₁ (1) isfrequency-modulated to produce -1st order diffracted light L₁ (1, -1) offrequency of f₀ +f₁ -f₂ (=F₁) and the beam L₂ (-1) isfrequency-modulated to produce first order diffracted light L₂ (-1, 1)of frequency of f₀ -f₁ +f₂ (=F₂). The beams (L₁ (1, -1), L₂ (-1, 1))leave the AOM 17 symmetrically at angle Φ₃ equal to the incident angleΦ₃.

With diffraction angle θ_(b2) (=2Φ₃) by acoustic Bragg diffraction inAOM 60, velocity v₂ of ultrasonic wave (traveling wave) crossing the AOM60, ultrasonic wave frequency f₂ of high-frequency signal SF₂,wavelength of light λ, and wavelength Λ₂ of ultrasonic wave (travelingwave) crossing the AOM 60, the following relations of Equations (15) and(16) may be established.

    Λ.sub.2 =v.sub.2 /f.sub.2                           (15)

    sin θ.sub.b2 =λ/Λ.sub.2                (16)

From above Equations (15) and (16), the diffraction angle θ_(b2) (=2Φ₃)by AOM 60 is finally obtained as in following Equation (17).

    sin θ.sub.b2 =f.sub.2 λ/v.sub.2 (or sin 2Φ.sub.3 =f.sub.2 λ/v.sub.2                                          (17)

If the second relay optical system (18a, 18b) has a magnification of β₂and the second relay optical system (18a, 18b) satisfies the sinecondition, the following relation of Equation (18) holds.

    β.sub.2 =(sin Φ.sub.2)/(sin Φ.sub.3)≈(sin 2Φ.sub.2)/(sin 2Φ.sub.3)                          (18)

Then, following Equation (19) may be derived from Equations (6), (17)and (18).

    β.sub.2 =(v.sub.2 f.sub.1)/(v.sub.1 f.sub.2)          (19)

As described, the second embodiment shown in FIG. 10 and FIG. 11 ispreferably arranged such that the first relay optical system (15a, 15b)satisfies above Equation (8) and that the second relay optical system(18a, 18b) satisfies above Equation (19).

If the first and second AOMs (17, 60) are made of the same material andif there is a frequency difference of several ten KHz between the firstand second high-frequency signals (f₁, f₂), β₂ ≈1 from above Equation(19), whereby the second relay optical system may be constructed to havea magnification of β₂ =1.

As described above, the second embodiment is arranged such that eachdetector (25, 33, 36) can photoelectrically detect beat light ofmultiple wavelengths including the predetermined frequency (Δf=|2 (f₁-f₂)|) (plural components of respective wavelengths are detected eachfrom the beat light including positional information of each diffractiongrating), so that high-precision alignment can be achieved in heterodyneinterference method while suppressing the influence of asymmetry ofdiffraction grating mark by averaging effect of beat light signals ofrespective wavelengths as well as the influence of thin filminterference of resist with multiple wavelength light. In addition, thesignal processing system can be simply constructed, because the beatfrequency can be greatly lowered.

Further, since the white light (multi-wavelength light) symmetricallysplit by the diffraction grating 14 (beam splitting means) with respectto the incident direction (direction of optical axis) travelssymmetrically and concurrently in the relay optical systems and theAOMs, there is no theoretical difference of optical path length betweenthe split beams. Therefore, the split beams have respective wavefrontsaligned with each other, that is, they have no phase difference, so thatthe apparatus may be easy in adjustment and compact in size as enablinghigh-precision alignment.

The second embodiment is so arranged that the diffraction light beamssymmetrically traveling in the first and second AOMs (17, 60) withrespect to the optical axis of second relay optical system (18a, 18b)are used as beams for alignment and that the signals of beat light ofpredetermined frequency (Δf=|2(f₁ -f₂)|), which are produced by applyingthe two diffracted light beams onto each of diffraction gratings (24,RM, WM) in two directions, are extracted by the detectors (25, 33, 36)and the optical beat signal extracting portion (Fourier transformcircuit) in phase difference detection system 50 to use the extractedsignals for alignment. Its reason is as follows.

Since noise light produced by the first AOM 17 is the same as in FIGS. 8and 9 as described above, it is omitted to explain here. Noise lightproduced by the second AOM 17 is described in the following withreference to FIGS. 12-15.

As shown in FIGS. 12-15, four beams of diffracted light produced by thefirst AOM 17 enter the second AOM 60, similarly as in FIGS. 8 and 9.

Specifically, as shown in FIG. 12, the beam L₁ (1) entering the secondAOM 60 (first order diffracted light of the beam L₁) is Bragg-diffractedby the second AOM 60 to produce -1st order diffracted light L₁ (1, -1)in optical path A and zeroth order diffracted light L₁ (1, 0) in opticalpath B.

As shown in FIG. 13, the beam L₂ (0) entering the second AOM 60 (zerothorder diffracted light of the beam L₂) is Bragg-diffracted by the secondAOM 60 to produce -1st order diffracted light L₂ (0, -1) in optical pathA and zeroth order diffracted light L₂ (0, 0) in optical path B.

As shown in FIG. 14, the beam L₂ (-1) entering the second AOM 60 (-1storder diffracted light of the beam L₂) is Bragg-diffracted by the secondAOM 60 to produce zeroth order diffracted light L₂ (-1, 0) in opticalpath A and +1st order diffracted light L₂ (-1, 1) in optical path B.

As shown in FIG. 15, the beam L₁ (0) entering the second AOM 60 (zerothorder diffracted light of the beam L₁) is Bragg-diffracted by the secondAOM 60 to produce zeroth order diffracted light L₁ (0, 0) in opticalpath A and +1st order diffracted light L₁ (0, 1) in optical path B.

There are frequencies of the diffracted light components leaving thesecond AOM 60, as listed below for each of the optical paths (A, B).

In optical path A:

frequency of -1st order diffracted light L₁ (1, -1) of beam L₁ (1): f₀+f₁ -f₂ (I);

frequency of -1st order diffracted light L₂ (0, -1) of beam L₂ (0): f₀-f₂ (II);

frequency of zeroth order diffracted light L₂ (-1, 0) of beam L₂ (-1):f₀ -f₁ (III);

frequency of zeroth order diffracted light L₁ (0, 0) of beam L₁ (0): f₀(IV).

In optical path B:

frequency of zeroth order diffracted light L₁ (1, 0) of beam L₁ (1): f₀+f₁ (I');

frequency of zeroth order diffracted light L₂ (0, 0) of beam L₂ (0): f₀(II');

frequency of first order diffracted light L₂ (-1, 1) of beam L₂ (-1): f₀-f₁ +f₂ (III');

frequency of first order diffracted light L.sub. (0, 1) of beam L.sub.(0): f₀ f₂ (IV').

The combination of the diffracted light components traveling in opticalpath A with the diffracted light components traveling in optical path Bproduces the following frequencies in beat light.

From the absolute value of difference between (I) and (I'),

    |(f.sub.0 +f.sub.1 -f.sub.2)-(f.sub.0 +f.sub.1)|=|f.sub.2                     [1].

From the absolute value of difference between (I) and (II'),

    |(f.sub.0 +f.sub.1 -f.sub.2)-f.sub.0 |=|f.sub.1 -f.sub.2 |                                       [2].

From the absolute value of difference between (I) and (III'),

    |(f.sub.0 +f.sub.1 -f.sub.2)-(f.sub.0 30 f.sub.2)|=|f.sub.1 -2(f.sub.2)|[3].

From the absolute value of difference between (I) and (IV'),

    |(f.sub.0 +f.sub.1 -f.sub.2)-(f.sub.0 +f.sub.2)|=|f.sub.1 2f.sub.2 | [4].

From the absolute value of difference between (II) and (I'),

    |(f.sub.0 -f.sub.2)-(f.sub.0 +f.sub.1)|=|f.sub.1 +f.sub.2 |                                       [5].

From the absolute value of difference between (II) and (II'),

    |(f.sub.0 -f.sub.2)-f.sub.0 |=|f.sub.2 |[6].

From the absolute value of difference between (II) and (III'),

    |(f.sub.0 -f.sub.2)-(f.sub.0 -f.sub.1 +f.sub.2)|=|f.sub.1 -2f.sub.2 |[7].

From the absolute value of difference between (II) and (IV'),

    |(f.sub.0 -f.sub.2)-(f.sub.0 +f.sub.2)|=|2f.sub.2 |         [8].

From the absolute value of difference between (III) and (I'),

    |(f.sub.0 -f.sub.1)-(f.sub.0 +f.sub.1)|=|2f.sub.1 |         [9].

From the absolute value of difference between (III) and (II'),

    |(f.sub.0 -f.sub.1)-f.sub.0 |=|f.sub.1 |[10].

From the absolute value of difference between (III) and (III'),

    |(f.sub.0 -f.sub.1)-(f.sub.0 f.sub.1 +f.sub.2)|=|f.sub.2 |          [11].

From the absolute value of difference between (III) and (IV'),

    |(f.sub.0 -f.sub.1)-(f.sub.0 +f.sub.2)|=|f.sub.1 +f.sub.2 |                                       [12].

From the absolute value of difference between (IV) and (I'),

    |f.sub.0 -(f.sub.0 +f.sub.1)|=|f.sub.1 |[13].

From the absolute value of difference between (IV) and (II'),

    |f.sub.0 -f.sub.0 |=0                    [14].

From the absolute value of difference between (IV) and (III'),

    |f.sub.0 -(f.sub.0 -f.sub.1 +f.sub.2)|=|f.sub.1 f.sub.2 |                                        [15].

From the absolute value of difference between (IV) and (IV'),

    |f.sub. -(f.sub.0 +f.sub.2)|=|f.sub.2 |[16].

Therefore, fifteen beat frequencies of [1]-[13], [15] and [16] are mixedin beat light photoelectrically detected by each of the detectors (25,33, 36). If the diffracted light includes the frequency [14] and isphotoelectrically detected by the detectors (25, 33, 36 ), it will be adirect current component (DC component). If it negatively affects thedetection precision, it can be removed upon Fourier transform in thebeat signal extracting portion in phase difference detection system 50.Alternatively, the direct current component may be removed by electricalfilter means separately provided.

Since a beat frequency which can be used for alignment is one which isunique in the above beat frequencies without doubly appearing therein,there is only one beat frequency of |2(f₁ -f₂)| produced by acombination of beam L.sub. (1, -1) with beam L₂ (-1, 1) in the secondembodiment. Thus, the optical beat signal extracting portion (Fouriertransform circuit) in phase difference detection system 50 extracts thesignal of beat frequency of |2(f₁ -f₂)|.

Even if beat light signals having various beat frequencies arephotoelectrically detected by the detectors (25, 33, 36), high-precisionalignment can be achieved by heterodyne interference based on the signalof predetermined beat frequency (|2(f₁ -f₂)|) extracted by the opticalbeat signal extracting portion (Fourier transform circuit).

If the second embodiment is so modified, as in the first embodiment,that at least either one of the diffraction grating 14 and the first AOM17 is rotated to make the traveling direction of traveling wave crossingthe first AOM 17 different from the pitch direction of diffractiongrating 14, the unnecessary diffracted light component L₂ (0) travelingin optical path A of beam L₁ (1) and the unnecessary diffracted lightcomponent L.sub. (0) traveling in optical path B of beam L₂ (-1) can beseparated out and then filtered by the space filter 19.

Further, if either one of the first AOM 17 and the second AOM 60 isrotated so that the traveling direction of traveling wave crossing thefirst AOM 17 is made different from that of traveling wave crossing thesecond AOM 60, the unnecessary diffracted light component L₂ (-1, 0)traveling in optical path A of beam L₁ (1, -1) and the unnecessarydiffracted light component L.sub. (1, 0) traveling in optical path B ofbeam L₂ (-1, 1) as shown in FIG. 12 and FIG. 13 can be separated out andfiltered by the space filter 61.

Even in case that the traveling direction of traveling wave crossing thefirst AOM 17 is made identical to that of traveling wave crossing thesecond AOM 60 and if the pitch direction of diffraction grating 14 ismade different from the traveling direction of traveling waves crossingthe two AOMs, the unnecessary diffracted light components can be alsoremoved by the space filters (19, 61 ), of course.

Now let us consider a general case that n AOMs are arranged in seriesand n relay optical systems are arranged for relay, one between the beamsplitting means (diffraction grating 14) and the first AOM and theothers between AOMs.

Let a frequency B_(f) be a frequency of beat light to be extracted.Since the diffracted light symmetrically traveling with respect to theoptical axis of each relay optical system can be used as illuminationbeams for alignment, the frequency B_(f) of beat light to be extractedis generally expressed by following Equation (20).

    B.sub.f =|2(f.sub.1 +f.sub.2 +f.sub.3. . . f.sub.n)|(20)

In the above equation, f_(n) represents a drive frequency of n-th AOMfrom the light source side, which is positive when the drive frequencyis applied in the first direction but negative when the drive frequencyis applied in the second direction opposite to the first direction.

Also, the magnification β₁ of first relay optical system is as definedin Equation (8), and a magnification β_(n) of n-th relay optical systemis defined by following Equation (21).

    β.sub.n =(v.sub.n f.sub.n-1)/(v.sub.n-1 -f.sub.n)     (21)

Although the second embodiment shown in FIG. 10-FIG. 15 shows such anexample that two beams of ±1st order diffracted light split by thediffraction effect of diffraction grating 14 are guided to enter thefirst AOM 17, that two beams, which are +1st order diffracted light fromone of the two beams diffracted through the first AOM 17 and -1st orderdiffracted light from the other of the two beams diffracted through thefirst AOM 17, are guided to enter the second AOM 60, and that the +1storder diffracted light from the one of two beams diffracted through thesecond AOM 60 and the -1st order diffracted light from the other of twobeams diffracted through the second AOM 60 are guided to irradiate marksfor position detection in two directions, the invention is not limitedto this example. For example, two diffracted light beams of arbitraryorder produced by the diffraction grating 14 may be used as two beamsfor position detection while guided to enter the first AOM 17. Also, adiffraction light beam of arbitrary order from one of two beamsdiffracted through the first AOM 17 and a diffracted light beam ofarbitrary order from the other of two beams diffracted through the firstAOM 17 can be used as two beams for position detection as guided toenter the second AOM 60. Further, a diffracted light beam of arbitraryorder from one of two beams diffracted through the second AOM 60 and adiffracted light beam of arbitrary order from the other of two beamsdiffracted through the second AOM 60 can be used as two beams forposition detection as guided to illuminate the marks for positiondetection in two directions.

Also, although the second embodiment shown in FIG. 10 shows an examplein which the light source means (10-12) supplies a beam including lightof plural wavelengths (multiple wavelengths), the invention is notlimited to this example. The alignment can be achieved in heterodyneinterference method with laser source supplying light of singlewavelength as light source means. In this case, the advantage in theheterodyne interference method using the multi-wavelength light cannotbe enjoyed in alignment. However, by arranging in series the diffractiongrating 14 and the first acousto-optic modulator 17 on either side ofthe first relay optical system (15a, 15b) and also by arranging inseries the first acousto-optic modulator 17 and the second acousto-opticmodulator 60 on either side of second relay optical system (18a, 18b),two beams different in frequency from each other as beat down can beproduced without difference of optical path length, whereby theapparatus may be simple in construction and remarkably easy inadjustment, as compared with the conventional apparatus.

Third Embodiment

The third embodiment according to the present invention will bedescribed with reference to FIG. 16. The present embodiment employs thethird type frequency difference producer as described before. In FIG.16, members having the same functions as those in the first embodimentshown in FIG. 1 are given the same reference numerals.

The present embodiment is different from the first embodiment in that anacousto-optic element 17a replaces the diffraction grating 14, the relayoptical system (condenser lens 15a, space filter 16 and condenser lens15b) and the AOM 17 in the first embodiment.

In the alignment optical system of the present embodiment, a white lightsource 10 is a light source such as a Xe lamp and a halogen lampsupplying light different in wavelength band from exposure light. Afterwhite light from the white light source 10 is converted into acollimated beam L₀ through an aperture-variable diaphragm 11 and acondenser lens 12, the beam is guided through a band-pass filter 13,which extracts light in predetermined wavelength range, then to enterthe AOM 17a in parallel with the wavefront of traveling wave.

The AOM 17a is driven by a high-frequency signal SF₁ of frequency f₁, sothat the beam L₀ in predetermined wavelength range is subjected to theRaman-Nath diffraction effect in AOM 17a.

Supposing the beam L₀ in predetermined wavelength range has a frequencyof f₀, the beam L₀ is frequency-modulated by AOM 17a to produce +1storder diffracted light L₀ (1) (as will be referred to as "beam L₀ (1)")of frequency (f₀ +f₁) and -1st order diffracted light L₀ (-1) (as willbe referred to as "beam L₀ (-1)") of frequency (f₀ -f₁).

After that, the beams L₀ (1) and L₀ (-1) will be treated in the samemanner as in the first embodiment.

Next described with FIG. 17 is a more specific structure of the portionfor producing two beams different in frequency from each other in thethird embodiment shown in FIG. 16, and the principle thereof.

FIG. 17 shows the acousto-optic modulator (AOM) 17a in FIG. 16. As shownin FIG. 17, the white light L₀ enters the AOM 17a in parallel with thewavefront of traveling wave therein. As a result, diffracted light ofvarious orders are produced from each wavefront by diffraction effect(Raman-Nath diffraction) of AOM 17a.

Letting a diffraction angle of N-th order diffracted light from incidentlight of wavelength λ be Φ₁ and a pitch of traveling wave be Λ₁, thefollowing equation holds.

    sin Φ.sub.1 =Nλ/Λ.sub.1                  (22)

The following equation holds as to the pitch Λ₁ of traveling wave withvelocity v₁ and frequency f₁ of traveling wave.

    Λ.sub.1 =v.sub.1 /f.sub.1                           (23)

Accordingly, Equation (22) may be rewritten into Equation (24) for ±1storder diffracted light.

    sin Φ.sub.1 =f.sub.1 λ/v.sub.1                  (24)

Now discussed is a diffraction angle of ±1st order light L₀ (1) and L₀(-1) passing through the space filter 19 in FIG. 16. For example, if thebase wavelength λ₀ of irradiation light is 633 nm, a width of wavelengthband ±50 nm, and the pitch Λ₁ of traveling wave in AOM 17a 40 μm, thenthe diffraction angle of ±1st order light is 0.835° for shortestwavelength of 583 nm and 0.978° for longest wavelength of 683 nm. Thus,with light of 583-683 nm the diffraction angle of ±1st order light isdistributed in a range of 0.835°-0.978°. As the incident light isdiffracted by the traveling wave in AOM 17, the diffracted light ismodulated by the amount of frequency of traveling wave therein.

Next described with FIG. 18 is the optical frequency modulation by theRaman-Nath diffraction. In FIG. 18 the angle θ_(i) is 0° between theincident light and the wavefront of traveling wave of ultrasonic wavecaused by the drive signal SF₁ in AOM 17a. Further, the angle is θ_(d)between the diffracted light and the wavefront of traveling wave ofultrasonic wave caused by the drive signal SF₁ in AOM 17a, a wave vectorof incident light into AOM 17a <K_(i) >, a wave vector of diffractedlight by AOM 17a <K_(d) >, and a wave vector of ultrasonic wave by drivesignal SF₁ <K_(s) >.

In the Raman-Nath diffraction, the angle θ_(d) is small, so that thevectors are in relation of isosceles triangle as shown in FIG. 18. If awavelength of light is λ, if an index of diffraction of AOM 17a is n, ifa frequency of ultrasonic wave is f₁, and if a velocity of ultrasonicwave (traveling wave) crossing the AOM 17a is v₁, then amplitudes ofvectors <K_(i) >, <K_(d) >and <K_(s) >may be expressed as follows.

    |<K.sub.i >|=2πn/λ             (25)

    |<K.sub.d >|=2πn/λ             (26)

    |<K.sub.s >|=2πf.sub.1 /v.sub.1       (27)

With wavelength Λ₁ of traveling wave of ultrasonic wave, the followingrelations hold.

    sin θ.sub.d =λ/Λ.sub.1                 (28)

    |<K.sub.s >|=sin θ.sub.d ·|<K.sub.d >|                  (29)

Equation (30) may be derived from Equations (28) and (29).

    |<K.sub.s >|=2πn/Λ.sub.1       (30)

As seen from Equation (30), the amplitude of |<K_(s) >| is constantirrespective of wavelength of light as far as the condition ofRaman-Nath diffraction is satisfied. It can be, therefore, understoodthat light which is to be diffracted by AOM 17a is subjected to the samefrequency modulation (f₁) regardless of wavelength of light. Thus, ifthe frequency of beam L₀ incident into AOM 17a is f, the +1st orderdiffracted light L₀ (1) from beam L₀ is equally frequency-modulated as(f+f₁) (=F₁) for each wavelength and the -1st order diffracted light L₀(-1) from the beam L₀ is also equally frequency-modulated as (f-f₁)(=F₂) for each wavelength. As described, the +1st order diffracted lightL₀ (1) of frequency F₁ having a certain wavelength range and the -1storder diffracted light L₀ (-1) of frequency F₂ having a certainwavelength range can be used to irradiate the diffraction gratings (24,RM, WM) symmetrically with light component of each wavelength at anincident angle different from others, so that ±1st order diffractedlight of each wavelength can be always produced in the direction normalto each of the diffraction gratings (24, RM, WM). Consequently, beatlight including a predetermined frequency Δf (=|F₁ -F₂ |=2f₁ |) may beproduced from the ±1st order diffracted light of each wavelength.Accordingly, since each detector (25, 33, 36) can photoelectricallydetect the beat light of multiple wavelengths including thepredetermined frequency Δf (= |F₁ -F₂ |=|2f₁ |) (beat light signals ofplural wavelengths including positional information of each diffractiongrating are detected), high-precision alignment can be achieved inheterodyne interference method while suppressing the influence ofasymmetry of diffraction grating mark by averaging effect of beat lightsignals of multiple wavelengths and the influence of thin filminterference of resist (influence such as a change in light quantity)with the multi-wavelength light.

In addition, the light diffracted by AOM 17a then travels symmetricallywith respect to the optical axis, so that there is no theoreticaldifference of optical path length between the split beams. In otherwords, the double-beam interference becomes possible even with whitelight short in interference distance. Also, since the wavefronts of thesplit beams are aligned with phase difference being zero, the apparatuscan be easy in adjustment and compact in size while enablinghigh-precision alignment.

The present embodiment is so arranged that the beams of ±1st orderdiffracted light L₀ (1), L₀ (-1) symmetrically leaving and opticallymodulated by AOM 17a are used as beams for alignment, the ±1st orderdiffracted light L₀ (1), L₀ (-1) are guided to irradiate the diffractiongratings (24, RM, WM) in two directions to obtain signals of beat lightof predetermined frequency Δf (=|2f₁), and the signals are extractedthrough the detectors (25, 33, 36) and the optical beat signalextracting portion (Fourier transform circuit) in phase differencedetection system 50 to use the extracted signals as signals foralignment. As so arranged, high-precision alignment can be achieved byheterodyne interference.

Although the present embodiment shows an example in which the ±1st orderdiffracted light beams split and produced by the Raman-Nath diffractioneffect of AOM 17a are used as two beams for position detection as toirradiate the marks for position detection in two directions, theinvention is not limited to this example. For example, any twodiffracted light components of arbitrary order produced by theacousto-optic modulator 17a may be used as two beams for positiondetection as to irradiate the marks for position detection in twodirections.

Fourth Embodiment

The fourth embodiment of the present invention will be describedreferring to FIG. 19 and FIG. 20. The present embodiment employs thefourth type frequency difference producer as described before. In FIG.19, members having the same functions as those in the first embodimentshown in FIG. 1 and in the third embodiment shown in FIG. 16 are giventhe same reference numerals, and the details thereof will be omitted toexplain. The present embodiment is different from the third embodiment,as the second embodiment is different from the first embodiment, in thata second acousto-optic modulator 60 (as will be referred to as "AOM 60")is provided between the relay optical system (18a, 18b) and thecondenser lens 21, that a space filter 61 is provided between thecondenser lens 21 and the beam splitter 22, and that the lenses 18a and18b serve as a relay optical system for relaying the diffraction pointof first AOM 17a (first acousto-optic modulator) to the diffractionpoint of second AOM 60 (second acousto-optic modulator).

In the present embodiment, a high-frequency signal SF₂ applied to thesecond AOM 60 travels in the direction opposite to the travelingdirection of high-frequency signal SF₁ applied to the first AOM 17a soas to lower beat frequency finally obtained (below 1 MHz), wherebyfacilitating the processing of electric signals.

As shown in FIG. 19, white light from white light source 10 supplyinglight in a wavelength band (multiple wavelengths) different from that ofexposure light is guided to pass through a variable aperture 11, acondenser lens 12 and a band-pass filter 13 then to enter the first AOM17a. The incident light is guided to enter the first AOM 17a in parallelwith the wavefront of traveling wave so that the Raman-Nath diffractionis effected in AOM 17a.

Since the first AOM 17a is driven by the first high-frequency signal SF₁of frequency f₁, a beam L₀ (1), which is ±1st order diffracted light ofthe beam L₀ in predetermined wavelength range, and a beam L₀ (-1), whichis -1st order diffracted light of beam L₀, are subjected to frequencymodulation of (f₀ +f₁) and (f₀ -f₁), respectively, in the first AOM 17a.After that, the beams L₀ (1) and L₀ (-1) are guided to pass through alens 18a, a reflection mirror 20 and a lens 18b then to enter the secondAOM 60 symmetrically at equal incident angle. A space filter 19 disposedin the relay optical system (18a, 18b) extracts ±1st order diffractedlight beams L₀ (1), L₀ (-1) leaving first AOM 17a.

The present embodiment is so arranged that the beams L₀ (1) and L₀ (-1)entering the second AOM 60 satisfy the condition of acoustic Braggdiffraction. Since the second AOM 60 is driven by the secondhigh-frequency signal SF₂ of frequency f₂ moving in the directionopposite to the traveling direction in first AOM 17a, -1st orderdiffracted light L.sub. (1, -1) (as will be referred to "beam L₀ (1,-1)") of the beam L₀ (1) in predetermined wavelength range is subjectedto frequency modulation of (f₀ +f₁ -f₂) (=F₁) by the second AOM 60, and+1st order diffracted light L₀ (-1, 1) (as will be referred to as "beamL₀ (-1, 1)") of the beam L₀ (-1) in predetermined wavelength range isalso subjected to frequency modulation of (f₀ -f₁ +f₂) (=F₂) by thesecond AOM 60. Then, the beams L₀ (1, -1) and L₀ (-1, 1) respectivelypass through a lens 21 and are then split into two by a beam splitter22. The space filter 61 is provided in the relay optical system (21, 23)to extract the -1st order diffracted light L₀ (1, -1) and the +1st orderdiffracted light L₀ (-1, 1) leaving from second AOM 60.

The two beams thus split by the beam splitter 22 are photoelectricallydetected finally by the detectors (25, 33, 36) in the same manner as inthe third embodiment shown in FIG. 16, the details of which are omittedto explain herewith. In the present embodiment, from photoelectricsignals photoelectrically detected by each detector (25, 33, 36), beatsignals of predetermined frequency Δf (=|F₁ -F₂ |=2(f₁ -f₂)|) arisingfrom a light component of each wavelength are extracted by the opticalbeat signal extracting portion (Fourier transform circuit) in phasedifference detection system 50 to carry out alignment based on thesesignals.

As described above, the present embodiment is so arranged that the twoAOMs 17a, 60 are arranged in series and that the high-frequency signalsSF₁, SF₂ are applied to the AOMs in the opposite directions, wherebyenabling the frequency of beat signals photoelectrically detected by thedetectors (25, 33, 36) to be lowered below 1 MHz, which is easy insignal processing.

Supposing the first AOM 17a is driven by a first high-frequency signalSF.sub. of frequency f₁ of 100 MHz and the second AOM 60 is driven by asecond high-frequency signal SF₂ of frequency f₂ of 99.9 MHz in thedirection opposite to that in first AOM 17a, the frequency of one beamL₀ (1, -1) passing through the second AOM 60 is F₁ (=f₀ +f₁ -f₂) and thefrequency of the other beam L₀ (-1, 1) passing through the second AOM 60is F₂ (=f₀ -f₁ +f₂), as described above. Then, the frequency Δf of beatlight, which is produced by each diffraction grating (24, RM, WM) withirradiation of the two beams L₀ (1, -1) and L₀ (-1, 1), will be 200 KHz(=|F₁ -F₂ |=|2(f₁ -f₂)|), which is a beat frequency easy in signalprocessing.

Next described with FIG. 20 is a more specific structure of the portionfor producing two beams different in frequency from each other in thepresent embodiment shown in FIG. 19.

As shown in FIG. 20, the present embodiment is so arranged that thefirst AOM 17a and the second AOM 60 are arranged in series and that therelay optical system (18a, 18b) is provided for relaying the diffractionpoint of first AOM 17a (in traveling path of high-frequency signal SF₁)to the diffraction point of second AOM 60 (in traveling path ofhigh-frequency signal SF₂). The beam L₀ of white light (multi-wavelight) enters the first AOM 17a in parallel with the wavefront oftraveling wave therein. Then, the beam L₀ is subjected to the Raman-Nathdiffraction by first AOM 17a.

Since the first AOM 17a is driven by the first high-frequency signal f₁,the ±1st diffracted light L₀ (1) of the beam L₀ is frequency-modulatedto have a frequency (f₀ +f₁) and the -1st diffracted light L₀ (-1) ofthe beam L₀ is frequency-modulated to have a frequency (f₀ -f₁). Thebeams L₀ (1) and L₀ (-1) leave the first AOM 17a as inclinedsymmetrically at angle Φ₁ with respect to the incident optical axis andas also being symmetric with respect to the wavefront of traveling wave.

The beams 1₀ (1) and L₀ (-1) optically modulated by first AOM 17a arecondensed by the first relay optical system (18a, 18b) then to enter thesecond AOM 60 symmetrically at angle Φ₃ with respect to the direction ofoptical axis. Since the second AOM 60 is driven by the secondhigh-frequency signal f₂ in the direction opposite to that in the firstAOM 17a, -1st diffracted light L₀ (1, -1) of the beam L₀ (1) isfrequency-modulated to have a frequency (f₀ +f₁ -f₂) (=F₁) and +1storder diffracted light L₀ (-1, 1) of the beam L₀ (-1) isfrequency-modulated to have a frequency (f₀ -f₁ +f₂) (=F₂). The beams L₀(1, -1) and L₀ (-1, 1) leave the second AOM 60 symmetrically at angle Φ₃equal to the incident angle Φ₃. In other words, the beams L₀ (1, -1) andL₀ (-1, 1) are respectively subjected to the acoustic Bragg diffractionin the second AOM 60.

If the diffraction angle by the acoustic Bragg diffraction of second AOM60 is θ_(b2) (=2Φ₃), if a velocity of ultrasonic wave (traveling wave)crossing the second AOM 60 is v₂, if an ultrasonic wave frequency ofhigh-frequency signal SF₂ is f₂, if a wavelength of light is λ, and if awavelength of ultrasonic wave (traveling wave) crossing the second AOM60 is Λ₂, then the following relations of Equations (31) and (32) hold.

    Λ.sub.2 =v.sub.2 /f.sub.2                           (31)

    sin θ.sub.b2 =λ/Λ.sub.2                (32)

From Equations (31) and (32), the diffraction angle θ_(b2) (=2Φ₃) by thesecond AOM 60 is finally obtained as in following Equation (33).

    sin θ.sub.b2 =f.sub.2 λ/v.sub.2 (or sin 2Φ.sub.3 =f.sub.2 λ/v.sub.2)                                         (33)

If the relay optical system (18a, 18b) has a magnification of β₁ andsatisfies the sine condition, the following relation of Equation (34)holds. ##EQU1##

Accordingly, following Equation (35) is derived from Equations (24),(33) and (34).

    β.sub.1 =2·(v.sub.2 f.sub.1)/(v.sub.1 f.sub.2)(35)

Therefore, the present embodiment shown in FIG. 19 and FIG. 20 ispreferably constructed such that the relay optical system (18a, 18b)satisfies above Equation (35).

If the first and second AOMs (17a, 60) are made of the same material andif there is a frequency difference of several ten KHz between the firstand the second high-frequency signals (f₂, f₁), β₁ in Equation (35) iscalculated as β₁ ≈2. Thus, the relay optical system (18a, 18b) may beconstructed with magnification β₁ equal to 2.

According to the present embodiment, as described above, each detector(25, 33, 36) can photoelectrically detect beat light of multiplewavelengths including the predetermined frequency Δf (=|2(f₁ -f₂)|) (bydetecting plural beat light signals of respective wavelengths includinginformation on position of each diffraction grating), so thathigh-precision alignment can be achieved in heterodyne interferencemethod while Suppressing the influence of asymmetry of diffractiongrating mark by averaging effect of beat light signals of variouswavelengths and the influence of thin film interference of resist onwafer 4 with multi-wavelength light. In addition, the beat frequency canbe greatly lowered, whereby the signal processing system can besimplified.

Further, the white light (multi-wavelength light) travels symmetricallyand concurrently through the relay optical systems and the AOMs, so thatthere is no theoretical difference of optical path length between thesplit beams. Therefore, the double-beam interference is possible evenwith a beam having a short coherence length such as white light. Also,since the wavefronts of the split beams are aligned, that is, since theyhave no phase difference, the apparatus can be constructed as easy inadjustment and compact in size while enabling the high-precisionalignment.

Fifth Embodiment

The fifth embodiment is constructed by the same members as in the firstembodiment as shown in FIG. 1. The fifth embodiment is different fromthe first embodiment in that the traveling direction of acoustic signal(SF₁) applied to the acousto-optic modulator (AOM) 17 is changed.

In case that the traveling direction of traveling wave in AOM 17 is setin parallel with the pitch direction of diffraction grating 14 as in thefirst embodiment, the zeroth order diffracted light is mixed in the ±1storder diffracted light so as to produce a beat signal of unnecessaryfrequency.

In order to avoid the unnecessary beat signal, the present embodiment isso arranged, as shown in FIG. 21, that the traveling direction oftraveling wave crossing the AOM 17 with high-frequency signal for driveis made different from the pitch direction of diffraction grating 14(orientation of gratings).

FIG. 21 shows the actual arrangement of optical members from thediffraction grating 14 and the space filter 19 in FIG. 1. In FIG. 21,the traveling direction of traveling wave in AOM 17 is deviated by apredetermined angle to the pitch direction of diffraction grating 14(diffraction direction). By this arrangement, unnecessary zeroth orderdiffracted light beams L₂ (0) and L.sub. (0) leave the AOM 17 in thedirection different from that of the diffracted light L₁ (1) or L₂ (-1),so that the space filter 19 can remove only the unnecessary zeroth orderdiffracted light components L₂ (0) and L₁ (0). In this case, it isnecessary for a diffraction angle of the beams L₁, L₂ split by thediffraction grating 14 to have a directional component in the directionof traveling wave in AOM 17 (Bragg angle component) equivalent to theBragg angle at AOM 17.

If the diffraction angle by the diffraction grating 14 is Φ₁ and if theangle is α between the diffraction direction by the diffraction grating14 (direction of a plane including diffracted light of respective ordersproduced by the diffraction grating 14) and the direction of travelingwaves in AOM 17, the above Bragg angle component is defined as follows.

    Bragg angle component=sin 2Φ.sub.1 ·cos α(36)

Accordingly, the pitch P_(G) of diffraction grating 14 must be (cos α)times smaller than that in case that the traveling direction oftraveling wave in AOM 17 is parallel to the beam split direction. Thus,Equation (8) should be modified as follows.

    β.sub.1 =(2v.sub.1 ·cos α)/(P.sub.G f.sub.1)(37)

The beams L₁ (1) and L₂ (-1) modulated by AOM 17 make a flow ofinterference fringes, which are identical to those obtained by rotatingthe original diffraction grating by 2α. Therefore, if the diffractiondirection of diffraction grating 14 is parallel to the plane of FIG. 1and if the traveling direction of traveling wave in AOM 17 is inclinedat angle α of 45° to the diffraction direction of diffraction grating14, the beams L₁ (1) and L₂ (-1) modulated by AOM 17 will be separatedin the direction normal to the plane of FIG. 1. Since these beams L₁ (1)and L₂ (-1) include no unnecessary beams, the position detection of thediffraction gratings RM and WM can be carried out at extremely high SNratio.

As seen from Equation (37), the magnification β₁ of relay optical system(15a, 15b) can be adjusted by changing α, that is, by rotating thediffraction grating 14 relative to the AOM 17.

Sixth Embodiment The sixth embodiment is obtained by modifying the fifthembodiment like the modification from the first embodiment to the secondembodiment while adding the same additional members as in themodification. Specifically, the sixth embodiment is constructed as shownin FIG. 22 and is different from the fifth embodiment in that theacoustic signal (SF₁) applied to AOM 17 travels in the directiondifferent from the traveling direction of the acoustic signal (SF₂)applied to the AOM 60.

In the present embodiment, as shown in FIGS. 22 and 23, the travelingdirection of traveling wave in AOM 17 is twisted to the pitch directionof .diffraction grating 14 similarly as in the fifth embodiment, and thetraveling direction of traveling wave in AOM 17 is also twisted to thetraveling direction of traveling wave in AOM 60, whereby mixture ofunnecessary noise light is prevented.

FIG. 22 shows the arrangement of optical members from diffractiongrating 14 to space filter 19, and FIG. 23 shows the arrangement ofoptical members from relay lens 18b to space filter 61. The arrangementin FIG. 22 is the same as in the fifth embodiment and is omitted toexplain herein.

In FIG. 23, the two ±1st order beams L₁ (1) and L₂ (-1) leaving thespace, filter 19 in FIG. 22 include no noise light of other orders, asdescribed in the fifth embodiment. In the present embodiment, thetraveling direction of traveling wave in AOM 60 is made different fromthe cross direction of incident beams, which is a direction normal tothe optical axis within a plane including the beams L₁ (1) and L₂ (-1).

Similarly as in the fifth embodiment, letting the angle be γ between theplane including the beams L₁ (1) and L₂ (-1) and the traveling directionof traveling wave in AOM 60, Equation (19) for magnification β₂ ofsecond relay optical system (18a, 18b) should be modified as follows.

    β.sub.2 =(v.sub.2 f.sub.1 cos γ)/(v.sub.1 f.sub.2)

The direction of -1st order diffracted light beam L.sub. (1, -1) of thebeam L₁ (1) from AOM 60 is different from the direction of zeroth orderdiffracted light L₂ (-1, 0) of the beam L₂ (-1) from AOM 60. Also, thedirection of +1st order diffracted light beam L.sub. (-1, 1) of the beamL₂ (-1) from AOM 60 is different from the direction of zeroth orderdiffracted light L.sub. (1, 0) of the beam L₁ (1) from AOM 60. Thus, thespace filter 61 can extract only the beams L₁ (1, -1) and L₂ (-1, 1).The pitch direction of interference fringes (cross direction) from thebeams L₁ (1, -1) and L₂ (-1, 1) is one obtained by rotating the pitchdirection of interference fringes from the incident beams L₁ (1) and L₂(-1) by angle 2γ.

Therefore, the pitch direction of interference fringes from the beamsL.sub. (1, -1) and L.sub. (-1, 1) in FIG. 23 is rotated by angle 2(α+γ)to the pitch direction of diffraction grating 14 in FIG. 22. If α=γ=45°for example, the pitch direction of interference fringes from the beamsL.sub. (1, -1) and L₂ (-1, 1).conveniently becomes parallel to the pitchdirection of diffraction grating 14.

The present embodiment shows an example in which the two AOMs 17, 60 arearranged in series. In the same manner, n (integer over 2) AOMs may bearranged in series in a general case. Also in such a case, two finalbeams can be obtained with a desired frequency difference but withoutunnecessary light by subsequently differing the traveling direction oftraveling wave in AOM between adjacent AOMs and removing unnecessaryfrequency beams by space filters. The other arrangement and theoperation of the sixth embodiment are the same as in the fifthembodiment.

In the above-described embodiments, the light source means was composedof the white light source 10 such as the Xe lamp and the halogen lamp,the variable aperture 11 and the condenser lens 12, and the white lightL₀ (multi-wavelength light) from the light source means is guided toimpinge normally onto the diffraction grating 14 (beam splitting means)or AOM 17a. The light source means may be one as shown in FIG. 24, inwhich a plurality of lasers (100, 101, 102 ) emit beams different inwavelength from each other to irradiate a brazed diffraction grating 103having a saw tooth section at respective incident angles different fromeach other, whereby a synthesized beam is obtained from the beams ofwavelengths emitted from the lasers (100, 101, 102).

The diffraction grating 14 in the first, the second, the fifth or thesixth embodiment is preferably constructed by a phase-type grating tosatisfy following equation (39):

    d=λ(M+0.5)/(n.sub.G -1),                            (39)

where d is a step of the phase-type diffraction grating 14, n is anindex of refraction of the diffraction grating 14, λ is a wavelength oflight and M is an integer. In this arrangement the diffractionefficiency may be remarkably enhanced without production of zeroth orderlight.

Although the above first, second, fifth, or sixth embodiment is soarranged that the white light (multi-wavelength light) supplied from thelight source means (10-12) is symmetrically split with respect to theincident direction (direction of optical axis) by the diffractiongrating 14 as beam splitting means, beam splitting means other than thediffraction grating 14, for example a Wollaston prism 140 as shown inFIG. 25, may be also employed. Alternatively, a reflection-typediffraction grating may be also employed as the diffraction grating 14.

Also, although each of the above embodiments is so arranged that anoptical beat signal extracting portion in phase difference detectionsystem 50 extracts a beat signal of predetermined frequency from eachphotoelectric signal photoelectrically detected by each detector (25,33, 36 ), an optical beat signal extracting portion (Fourier transformcircuit) may be provided in an electric path between each detector (25,33, 36) and the phase difference detection system 50 so that aphotoelectric signal photoelectrically detected by each detector (25,33, 36 ) is independently Fourier-transformed.

Although the alignment is carried out in each of the above embodimentsin the heterodyne interference method utilizing the two beams of ±1storder diffracted light illuminating the alignment marks (RM, WM) in twodirections for diffraction, the alignment may also be performed inheterodyne interference method by another arrangement as disclosed forexample in Japanese Laid-open Patent Application No. 2-133913, in whichthe pitch of alignment marks (RM, WM) is a half of that in eachembodiment and detection light is a beam of zeroth order lightilluminating the alignment marks (RM, WM) and a beam of +2nd orderdiffracted light (or -2nd order diffracted light) illuminating thealignment marks (RM, WM). Further, the alignment by heterodyneinterference method is also possible by the arrangement as disclosed inJapanese Laid-open Patent Application No. 4-7814, in which the detectionlight is a diffracted light beam appearing in the direction opposite toa first direction in which a first beam illuminates an alignment mark RMon reticle and a diffracted light beam appearing in the directionopposite to a second direction, different from the first direction, inwhich a second beam illuminates the alignment mark RM on reticle, and inwhich the two diffracted detection light beams are made interfering witheach other by a diffraction grating disposed conjugate with a wafer inalignment optical system so that the interference light is detected by adetector.

Also, in case that the light source is the plural laser sources as shownin FIG. 24, an arrangement may be such that laser beams different inwavelength from each other impinge on a diffraction grating 14' atrespective incident angles different from each other and that a spacefilter 16' extracts zeroth order light and +1st order diffracted light(or -1st order diffracted light).

For example, supposing laser beams of wavelengths λ₄ and λ₅ impinge onthe diffraction grating 14' at respective incident angles Φ₄ and Φ₅, theincident angles are arranged to satisfy the following equations.

sin (Φ₄ /2)=λ₄ /P_(G) '

sin (Φ₅ /2)=λ₅ P_(G) '

In the equations P_(G) ' represents the pitch of diffraction grating14'. By this arrangement, there are angles 2Φ₄ and 2Φ₅ between thezeroth order light beam and the first order diffracted light beam forthe respective laser beams as the beams are symmetric with respect tothe normal line to the diffraction grating 14'. Since there is nounnecessary light between the laser beams passing through the spacefilter 16', the assembling precision of space filter 16' may be relaxedas compared with the space filter 16 extracting the ±1st orderdiffracted light as shown in FIG. 1.

Also, the diffraction grating 14 in the first, second, fifth or sixthembodiment may be replaced by an acousto-optic modulator for effectingthe Raman-Nath diffraction.

FIG. 27 shows an example in which the diffraction grating 14 in thefifth embodiment as shown in FIG. 1 is replaced by an acousto-opticmodulator 70 (as will be referred to as "AOM 70"). In this example, thedirection of high-frequency signal SF₃ applied to the AOM 70 is oppositeto that of high-frequency signal SF₁ applied to the AOM 17 so that afinally obtained beat frequency may be lowered (below 1 MHz),facilitating the processing of electric signals. In this example thetraveling direction of traveling wave in AOM 70 and the travelingdirection of traveling wave in AOM 17 are not parallel to each other,but they are illustrated in parallel in FIG. 28 and FIG. 29 forconvenience of description.

FIG. 27 shows the schematic structure of a projection exposure apparatusin this example. In FIG. 27, a beam L₀ from white light source 10 isguided to pass through a variable aperture 11, a condenser lens 12 and aband-pass filter 13 then to enter the AOM 70 driven by a high-frequencysignal SF₃ of frequency f₃ in parallel with the wavefront of travelingwave in the AOM. Accordingly, a +1st order diffracted light beam L₀ (1)and a -1st order diffracted light beam L₀ (-1) are produced byRaman-Nath diffraction from the AOM 70.

After that, the beams L₀ (1) and L₀ (-1) are guided to pass through alens 15a, a space filter 16 and a lens 15b then to enter the AOM 17symmetrically at the same incident angle. A space filter 16 extracts the±1st order diffracted light L₀ (1), L₀ (-1) leaving AOM 70. This exampleis arranged such that the beams L₀ (1) and L₀ (-1) entering the AOM 17satisfy the condition of acoustic Bragg diffraction. Since the AOM 17 isdriven by a second high-frequency signal SF₁ of frequency f₁ in theopposite direction to that in AOM 70, -1st order diffracted light L₀ (1,-1) (as will be referred to as "beam L₀ (1, -1)") form the beam L₀ (1)in predetermined wavelength range is frequency-modulated by AOM 17 tohave a frequency (f₀ +f₃ -f₁) (=F₁) and +1st order diffracted light L₀(-1, 1) (as will be referred to as "beam L₀ (-1, 1)") from the beam L₀(-1) in predetermined wavelength range is also frequency-modulated byAOM 17 to have a frequency (f₀ -f₃ +f₁) (=F₂).

Then, the beams L₀ (1, -1) and L₀ (-1, 1) are guided through a relayoptical system (18a, 18b) and a lens 21 and then are respectively splitinto two by a beam splitter 22. A space filter 19 in relay opticalsystem (18a, 18b) extracts the -1st order diffracted light L₀ (1, -1)and the +1st order diffracted light L₀ (-1, 1) leaving the AOM 17. Thebeams thus two-split by the beam splitter 22 are finallyphotoelectrically detected by the detectors (25, 33, 36) in the samemanner as in the fifth embodiment, the details of which are thereforeomitted to explain herein.

As described above, this example is so arranged that the two AOMs 70, 17are arranged in series and that the AOMs are driven by thehigh-frequency signals SF₃ and SF.sub. opposite to each other, so thatthe frequency of beat signal photoelectrically detected by each detector(25, 33, 36) may be lowered below 1 MHz, which is easy in signalprocessing.

Next described with FIG. 28 is a more detailed structure of the portionfor producing two beams different in frequency from each other in thisexample shown in FIG. 27.

As shown in FIG. 28, this example is so arranged that the AOM 70 and theAOM 17 are arranged in series and that the relay optical system (15a,15b) is provided for relaying the diffraction point of AOM 70 (intraveling path of high-frequency signal SF₃) to the diffraction point ofAOM 17 (in traveling path of high-frequency signal SF₁). The beam L₀ ofwhite light (multi-wavelength light) enters the AOM 70 in parallel withthe wavefront of traveling wave therein and then is subjected to theRaman-Nath diffraction by AOM 70.

If N-th diffracted light from incident light of wavelength λ has adiffraction angle Φ₁ and if the pitch of traveling wave in AOM 70 is Λ₃,the following equation holds.

    sin Φ.sub.1 =Nλ/Λ.sub.3                  (40)

For the pitch Λ₃ of traveling wave, the following equation stands withvelocity v₃ and frequency f₃ of traveling wave.

    Λ.sub.3 =v.sub.3 /f.sub.3                           (41)

Accordingly, Equation (40) may be rewritten as follows for ±1st orderdiffracted light.

    sin Φ.sub.1 =f.sub.3 λ/v.sub.3                  (42)

Now discussed is the diffraction angle of ±1st order light L₀ (1), L₀(-1) passing through the space filter 16 in FIG. 28. For example,suppose the base wavelength λ₀ of irradiation light is 633 nm, the widthof wavelength band is ±50 nm, and the pitch Λ₃ of traveling wave in AOM70 is 40 μm. The diffraction angle of ±1st order light is 0.835° forshortest wavelength of 583 nm and 0.978° for longest wavelength of 683nm. Accordingly, with incident light of 583-683 nm the diffraction angleof ±1st order light is distributed within a range of 0.835°-0.978°.Since the beams are diffracted by the traveling wave in AOM 70, thediffracted light is frequency-modulated therein by the frequency oftraveling wave.

Also, it is found that the optical frequency modulation by theRaman-Nath diffraction is irrespective of the wavelength of incidentbeam, similarly as in the Bragg diffraction, in which the +1st orderdiffracted light is frequency-modulated by an amount of +f₃ and the -1stdiffracted light is frequency-modulated by an amount of -f₃. If the ±1storder diffracted light is utilized in this example, +1st orderdiffracted light L₀ (1) (as will be also referred to as "beam L₀ (1)")and -1st order diffracted light L₀ (-1) (as will be also referred to as"beam L₀ (-1)") leave the AOM 70 symmetrically with respect to theoptical axis of incident light, and the beams L₀ (1) and L₀ (-1) arefrequency-modulated by the AOM 70 to have frequencies (f₀ +f₁) and (f₀-f₁), respectively.

The beams L₀ (1) and L₀ (-1), which are frequency-modulated by AOM 70 tohave the frequencies (f₀ +f₃) and (f₀ -f₃), are inclined symmetricallyat angle Φ₁ with respect to the optical axis of incident light, leavingthe AOM 70 symmetrically with respect to the wavefront of travelingwave. Then, the beams L₀ (1) and L₀ (-1) are condensed by the relayoptical system (15a, 15b) to enter the AOM 17 symmetrically at angle Φ₃with respect to the direction of optical axis. -1st order diffractedlight L₀ (1, -1) from the beam L₀ (1) is frequency-modulated there tohave a frequency (f₀ +f₃ -f₁) (=F₁), while +1st order diffracted lightL₀ (-1, 1) from the beam L₀ (-1) is frequency-modulated to have afrequency (f₀ +f₃ -f₁) (=F₂). The beams L₀ (1, -1) and L₀ (-1, 1) leavethe AOM 17 symmetrically at angle Φ₃ equal to the incident angle Φ₃. Inother words, the beams L₀ (1, -1) and L₀ (-1, 1) are diffracted inacoustic Bragg diffraction by the AOM 17.

With diffraction angle by acoustic Bragg diffraction of AOM 17 beingθ_(b1) (=2Φ₃), velocity of ultrasonic wave (traveling wave crossing theAOM 17 being v₁, ultrasonic wave frequency of high-frequency signalSF.sub. being f₁, wavelength of light being λ, and wavelength ofultrasonic wave (traveling wave) crossing the AOM 17 being Λ₁, Equations(4) to (6) in the first embodiment hold. As for the magnification β₁ ofrelay optical system (15a, 15b), the relation of Equation (8) in thefirst embodiment also holds. In detail, the magnification β₁ of relayoptical system (15a, 15b) is expressed by Equation (43).

    β.sub.1 =2·f.sub.3 v.sub.1 /(f.sub.1 v.sub.3)(43)

If the two AOMs (70, 17) are made of the same material and if thefrequency difference is of several ten KHz between the third and thefirst high-frequency signals (f₃, f₁), β₁ in equation (43) becomesnearly equal to 2. Thus, the relay optical system (15a, 15b) may beconstructed to have the magnification β₁ equal to 2.

In the arrangement of FIG. 28, however, zeroth order light from the beamL₀ (1) is mixed in the beam L₀ (1, -1) leaving the AOM 17, and zerothorder light from the beam L₀ (1) is mixed in the beam L₀ (-1, 1). Inorder to avoid the mixture, the present example is so arranged, as shownin FIG. 29, that the traveling direction of traveling wave in front AOM70 is made different from the traveling direction of traveling wave inrear AOM 17.

FIG. 29 shows the actual arrangement of optical members from front AOM70 to space filter 19 shown in FIG. 27. In FIG. 29, the travelingdirection of traveling wave of front AOM 70 is intersecting at angle αwith the traveling direction of traveling wave of rear AOM 17.Accordingly, the -1st order diffracted light L₀ (1, -1) leaving the rearAOM 17 is separated from the zeroth order light L₀ (-1, 0) of the beamL₀ (-1) while the +1st order diffracted light L₀ (-1, 1) leaving the AOM17 is separated from the zeroth order light L₀ (1, 0) of the beam L₀(1). Then, the space filter 19 can extract only the beams L₀ (1, -1) andL₀ (-1, 1), which remarkably improves the SN ratio of detection signals.

The pitch Λ₃ of traveling wave in front AOM 70 is (cos α) times smallerthan that in case that the traveling directions of traveling waves inAOM 70 and AOM 17 are parallel to each other, similarly as in the fifthembodiment. The magnification β₁ of relay optical system (15a, 15b) isgiven for this arrangement by Equation (44).

    β.sub.1 =2·cos α·f.sub.3 v.sub.1 /(f.sub.1 v.sub.3)                                                  (44)

In the present embodiment, at least one of front AOM 70 and rear AOM 17is rotated to the other so that the traveling direction of travelingwave crossing the front AOM 70 is made different from the travelingdirection of traveling wave in rear AOM 17, whereby unnecessarydiffracted light may be surely optically removed. Since the front AOM 70utilizes the Raman-Nath diffraction, the two functions of beamseparation and provision of frequency difference between the separatebeams are achieved by a single AOM 70, whereby the structure ofapparatus may be simplified.

As seen from Equation (37), the magnification β₁ of relay optical system(15a, 15b) may be adjusted by changing α, or by rotating the AOM 70relative to AOM 17.

Further, two beams symmetrically enter and leave each of two AOMs 70,17, so that even if light including plural wavelengths is used for suchbeams, a difference of optical path length between the beams is verysmall when they finally impinge on the diffraction grating marks RM, WM,whereby the position detection may be well conducted. Therefore, thehigh-precision alignment can be achieved in heterodyne interferencemethod while suppressing the influence of asymmetry of diffractiongrating mark by averaging effect of beat light signals of somewavelengths and the influence of thin film interference of resist(influence of change in light quantity) with multi-wavelength light.

The acousto-optic elements (AOM 17, AOM 60) effecting the Braggdiffraction in the first, second, fifth and sixth embodiments may bereplaced by another means.

FIG. 30 shows a so-called radial grating as an alternative, in whichtransparent portions and shielding portions are alternately formed at acertain pitch on a disk, which is mechanically rotated through anunrepresented drive system.

FIG. 31 shows another alternative, in which on a device changing itsindex of refraction by voltage application (e.g., electro-opticmodulators, photochromic devices, liquid crystal devices, etc.)transparent electrodes are juxtaposed as opposing to each other oneither side of the device and in which the electrodes are sequentiallydriven. FIG. 31 shows an example in which three sets of transparentelectrodes correspond to a pitch of fringes (at least three sets ofelectrodes are theoretically necessary for fringe formation) and inwhich the electrodes are arranged at equal intervals such that adistance between the three sets of transparent electrodes corresponds toa pitch of fringes.

In this arrangement, a drive voltage of i-th transparent electrodes(a_(i1), a_(i2)) is as follows:

    v.sub.i =v.sub.0 cos [2π(f+i/3)t],

where

i: order of arrangement of electrode pair in set,

v_(i) : drive voltage,

v₀ : base voltage,

f: drive frequency,

t: time.

In case that the transparent electrodes are arranged such that N sets oftransparent electrodes corresponds to a pitch of fringes, a drivevoltage of i-th transparent electrodes (a_(i1), a_(i2)) is as follows:

    v.sub.i =v.sub.0 cos [2π(f+i/N)t],

where

i: order of arrangement of electrode pair in set,

v_(i) : drive voltage,

v₀ : base voltage,

f: drive frequency,

t: time.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A double-beam light source apparatus comprising:alight source system for supplying a beam; and a frequency differenceproducing system for receiving the beam outgoing from said light sourcesystem to split it into two beams and producing a predeterminedfrequency difference between said two beams to output said two beamsradially spreading, the frequency difference producing systemcomprisinga light splitting unit for splitting a beam incident thereintointo two beams crossing at a first predetermined position and radiallyspreading to output the two split beams; a first relay optical systemfor receiving the two beams outgoing from said beam splitting unit andconverging the two beams at a second predetermined position; a firstmodulating unit, disposed at said second predetermined position, formodulating a frequency of incident beam in accordance with an incidentdirection thereof and for changing an outgoing direction thereof; asecond relay optical system for receiving the two beams outgoing fromsaid first modulating unit and converging the two beams at a thirdpredetermined position; and a second modulating unit, disposed at saidthird predetermined position, for modulating a frequency of incidentbeam in accordance with an incident direction thereof and for changingan outgoing direction thereof, wherein transmission optical path lengthsof the two beams are identical to each other and the two beams havingsaid predetermined frequency difference are made separately outgoing. 2.A double-beam light source apparatus according to claim 1, wherein saidlight source system comprises a light source emitting a beam of singlewavelength and a condensing optical system for condensing the beam fromsaid light source.
 3. A double-beam light source apparatus according toclaim 1, wherein said light source system comprises a light source forgenerating a beam including light of plural wavelengths and a condensingoptical system for condensing the beam from said light source.
 4. Adouble-beam light source apparatus according to claim 1, wherein saidfirst relay optical system comprises a first lens group having apositive refractive power and a conjugate point of said first lens groupis at said second predetermined position and wherein said second relayoptical system comprises a second lens group having a positiverefractive power and a conjugate point of said second lens group is atsaid third predetermined position.
 5. A double-beam light sourceapparatus according to claim 1, wherein said first modulating unitcomprises a first acousto-optic modulator, which effects Braggdiffraction on incident light by a diffraction grating pattern formed bya compressional wave applied to said acousto-optic modulator, andwhereinsaid second modulating unit comprises a second acousto-optic modulator,which effects Bragg diffraction on incident light by a diffractiongrating pattern formed by a compressional wave of the second modulatorunit applied to said second acousto-optic modulator, said compressionalwave of the second modulating unit traveling in the same direction as atravelling direction of a diffraction grating pattern image formed atsaid third predetermined position when a diffraction grating pattern ofsaid first acousto-optic modulator is projected by said second relayoptical system.
 6. A double-beam light source apparatus according toclaim 5, wherein said compressional wave applied to said firstacousto-optic modulator travels in a direction approximately normal to adirection of a sum vector obtained from wave vectors of the two beamsincident into said first acousto-optic modulator, andwherein saidcompressional wave applied to said second acousto-optic modulatortravels in a direction approximately normal to a direction of a sumvector obtained from wave vectors of the two beams incident into saidsecond acousto-optic modulator.
 7. A double-beam light source apparatusaccording to claim 5, wherein the traveling direction of thecompressional wave applied to said first acousto-optic modulator crossesa plane including the wave vectors of two beams incident into said firstacousto-optic modulator, andwherein the traveling direction of thecompressional wave applied to said second acousto-optic modulatorcrosses a plane including the wave vectors of two beams incident intosaid second acousto-optic modulator.
 8. A double-beam light sourceapparatus according to claim 1, further comprising an optical unit forcollimating said two radially spreading beams outgoing from saidfrequency difference producing system.
 9. A position detecting apparatuscomprising:a double-beam light source apparatus comprising a lightsource system for supplying a beam; and a frequency difference producingsystem for receiving the beam outgoing from said light source system tosplit it into two beams and for producing a predetermined frequencydifference between said two beams to output said two beams radiallyspreading, in which transmission light path lengths of the two beams areidentical to each other and the two beams having said predeterminedfrequency difference are made separately outgoing, the frequencydifference producing system comprisinga) a light splitting unit forsplitting a beam incident thereinto into two beams crossing at a firstpredetermined position and radially spreading to output the two splitbeams; b) a first relay optical system for receiving the two beamsoutgoing from said beam splitting unit and converging the two beams at asecond predetermined position; c) a first modulating unit disposed atsaid second predetermined position, for modulating a frequency ofincident beam in accordance with an incident direction thereof and forchanging an outgoing direction thereof; d) a second relay optical systemfor receiving the two beams outgoing from said first modulating unit andconverging the two beams at a third predetermined position; and e) asecond modulating unit disposed at said third predetermined position,for modulating a frequency of incident beam in accordance with anincident direction thereof and for changing an outgoing directionthereof; a light separating unit for separating the two beams outgoingfrom said double-beam light source apparatus into two sets of doublebeams; a first condensing system for receiving a set of double beamsoutgoing from said light separating unit and condensing the set ofdouble beams at a reference diffraction grating, said referencediffraction grating disposed at a condensing position of said firstcondensing means; a first photodetector for detecting an opticalinformation resulting from diffraction by said reference diffractiongrating; a second condensing system for condensing the other set ofdouble beams outgoing from said light separating unit, the secondcondensing system condensing the other set of double beams on adiffraction grating formed on an object to be measured; a secondphotodetector for detecting an optical information resulting fromdiffraction by the diffraction grating; and an optical informationprocessing system for receiving an output signal of said firstphotodetector and an output signal of said second photodetector tocompare the both optical information with each other and for calculatinga displacement amount of said object to be measured from a referenceposition.
 10. A position detecting apparatus according to claim 9,wherein said light source system comprises a light source emitting abeam of single wavelength and a condensing optical system for condensingthe beam from said light source.
 11. A position detecting apparatusaccording to claim 9, wherein said light source means comprises a lightsource emitting a beam including light of plural wavelengths and acondensing optical system for condensing the beam from said lightsource.
 12. A position detecting apparatus according to claim 9, whereinsaid first relay optical system comprises a first lens group having apositive refractive power and a conjugate point of said first lens groupis at said second predetermined position, andwherein said second relayoptical system comprises a second lens group having a positiverefractive power and a conjugate point of said second lens group is atsaid third predetermined position.
 13. A position detecting apparatusaccording to claim 9, wherein said first modulating unit comprises afirst acousto-optic modulator, which effects Bragg diffraction onincident light by a diffraction grating pattern formed by acompressional wave applied to said acousto-optic modulator, andwhereinsaid second modulating unit comprises a second acousto-optic modulator,which effects Bragg diffraction on incident light by a diffractiongrating pattern formed by a compressional wave of the second modulatingunit applied to said second acousto-optic modulator, said compressionalwave of the second modulating unit traveling in a same direction as atravelling direction of a diffraction grating pattern image formed atsaid third predetermined position when a diffraction grating pattern ofsaid first acousto-optic modulator is projected by said second relayoptical system.
 14. A position detecting apparatus according to claim13, wherein said compressional wave applied to said first acousto-opticmodulator travels in a direction approximately normal to a direction ofa sum vector obtained from wave vectors of the two beams incident intosaid first acousto-optic modulator, andwherein said compressional waveapplied to said second acousto-optic modulator travels in a directionapproximately normal to a direction of a sum vector obtained from wavevectors of the two beams incident into said second acousto-opticmodulator.
 15. A position detecting apparatus according to claim 13,wherein a traveling direction of the compressional wave applied to saidfirst acousto-optic modulator crosses a plane including the wave vectorsof two beams incident into said first acousto-optic modulator,andwherein a traveling direction of the compressional wave applied tosaid second acousto-optic modulator crosses a plane including the wavevectors of two beams incident into said second acousto-optic modulator.16. A position detecting apparatus according to claim 9, furthercomprising:a third condensing system for re-condensing two divergingbeams after condensed by said second condensing system; and a thirdphotodetector, disposed at a condensing position of said thirdcondensing system, for detecting an optical information resulting fromdiffraction by a diffraction grating formed on a second object to bemeasured, wherein said optical information processing system receives anoutput signal of said third photodetector in addition to the outputsignals of said first and second photodetectors to obtain a relativepositional deviation between a position of said first object to bemeasured and a position of said second object to be measured.
 17. Analigning apparatus comprising:a position detecting apparatus as setforth in claims 16; a first drive unit for said first object to bemeasured; and a second drive unit for said second object to be measured,wherein said aligning apparatus performs alignment by adjusting thepositional deviation between the objects to be measured.
 18. An exposureapparatus comprising:an illumination optical system for illuminating anexposure light on a mask to replicate a predetermined pattern formed onsaid mask onto a substrate; a position detecting system for opticallydetecting positions of said mask and said substrate respectively; acalculating unit for calculating relative displacement between said maskand said substrate based on an output from said position detectingsystem; and a driving unit for relatively moving said mask and saidsubstrate based on an output from said calculating unit, wherein saidposition detecting system comprisesa double-beam light source apparatuscomprisinga light source system for supplying a beam, and a frequencydifference producing system for receiving the beam outgoing from saidlight source system to split it into two beams and for producing apredetermined frequency difference between said two beams to output saidtwo beams radially spreading, in which transmission light path lengthsof the two beams are identical to each other and the two beams havingsaid predetermined frequency difference are made separately outgoing,and wherein the frequency difference producing system comprisesa) alight splitting unit for splitting a beam incident thereinto into twobeams crossing at a first predetermined position and radially spreadingto output the two split beams; b) a first relay optical system forreceiving the two beams outgoing from said beam splitting unit andconverging the two beams at a second predetermined position; c) a firstmodulating unit disposed at said second predetermined position, formodulating a frequency of incident beam in accordance with an incidentdirection thereof and for changing an outgoing direction thereof; d) asecond relay optical system for receiving the two beams outgoing fromsaid first modulating unit and converging the two beams at a thirdpredetermined position; and e) a second modulating unit disposed at saidthird predetermined position, for modulating a frequency of incidentbeam in accordance with an incident direction thereof and for changingan outgoing direction thereof; a first optical condensing system forrespectively condensing two beams output from said double-beam lightsource apparatus on diffraction grating marks respectively formed onsaid mask and said substrate; a first photodetector for detecting alight grated by said diffraction grating formed on said substrate tooutput a first detection signal to said calculating unit; and a secondphotodetector for detecting a light grated by said diffraction gratingformed on said mask to output a second detection signal to saidcalculating unit.
 19. An exposure apparatus according to claim 18,further comprising a projection optical system for projecting saidpredetermined pattern on said mask on said substrate with apredetermined magnification.
 20. An exposure apparatus according toclaim 18, further comprising:a beam splitting unit for splitting the twobeams output from said double-beam light apparatus into two light beamsets; a second optical condensing system for receiving two beams in oneof the two light beam sets which are split by said beam splitting unit,and condensing said two beams at a standard detraction grating; and athird photo-detector for detecting a light grated in said standarddetraction grating to output a third detection signal to saidcalculating unit, said calculating unit calculating a relativedisplacement between said mask and said substrate.
 21. An exposureapparatus according to claim 18, wherein said driving meanscomprisesmask holding units for holding said mask and a substrateholding unit for holding said substrate, either one of said mask holdingunits and said substrate holding unit being moved based on an outputfrom said calculating unit.
 22. An exposure apparatus according to claim18, wherein said light source system comprises a light source emitting abeam of single wavelength and a condensing optical unit for condensingthe beam from said light source.
 23. An exposure apparatus according toclaim 18, wherein said light source system comprises a light sourceemitting a beam including light of plural wavelengths and a condensingoptical unit for condensing the beam from said light source.
 24. Anexposure apparatus according to claim 18, wherein said first relayoptical system comprises a first lens group having a positive refractivepower and a conjugate point of said first lens group is at said secondpredetermined position, andwherein said second relay optical systemcomprises a second lens group having a positive refractive power and aconjugate point of said second lens group is at said third predeterminedposition.
 25. An exposure apparatus according to claim 18, wherein saidfirst modulating unit comprises a first acousto-optic modulator, whicheffects Bragg diffraction on incident light by a diffraction gratingpattern formed by a compressional wave applied to said acousto-opticmodulator, andwherein said second modulating unit comprises a secondacousto-optic modulator, which effects Bragg diffraction on incidentlight by a diffraction grating pattern formed by a compressional wave ofthe second modulating unit applied to said second acousto-opticmodulator, said compressional wave of the second modulating unittraveling in a same direction as a travelling direction of a diffractiongrating pattern image formed at said third predetermined position when adiffraction grating pattern of said first acousto-optic modulator isprojected by said second relay optical system.
 26. An exposure apparatusaccording to claim 25, wherein said compressional wave applied to saidfirst acousto-optic modulator travels in a direction approximatelynormal to a direction of a sum vector obtained from wave vectors of thetwo beams incident into said first acousto-optic modulator, andwhereinsaid compressional wave applied to said second acousto-optic modulatortravels in a direction approximately normal to a direction of a sumvector obtained from wave vectors of the two beams incident into saidsecond acousto-optic modulator.
 27. An exposure apparatus according toclaim 25, wherein a traveling direction of the compressional waveapplied to said first acousto-optic modulator crosses a plane includingthe wave vectors of two beams incident into said first acousto-opticmodulator, andwherein a traveling direction of the compressional waveapplied to said second acousto-optic modulator crosses a plane includingthe wave vectors of two beams incident into said second acousto-opticmodulator.
 28. A double-beam light source apparatus comprising:a lightsource system for supplying a beam, said light source system having alight source for generating a beam including light of plural wavelengthsand a collimating optical system for condensing the beam from said lightsource; and frequency difference producing means for receiving the beamoutgoing from said light source means to split it into two beams andproducing a predetermined frequency difference between said two beams tooutput said two beams radially spreading; wherein transmission opticalpath lengths of the two beams are identical to each other and the twobeams having said predetermined frequency difference are made separatelyoutgoing.
 29. A position detecting apparatus comprising:a double-beamlight source apparatus comprisinga light source system for supplying abeam, the light source system having a light source emitting a beamincluding light of plural wavelengths and a condensing optical systemfor condensing the beam from said light source; and a frequencydifference producing system for receiving the beam outgoing from saidlight source system to split it into two beams and for producing apredetermined frequency difference between said two beams to output saidtwo beams radially spreading, in which transmission light path lengthsof the two beams are identical to each other and the two beams havingsaid predetermined frequency difference are made separately outgoing; alight separating unit for separating the two beams outgoing from saiddouble-beam light source apparatus into two sets of double beams; afirst condensing system for receiving a set of double beams outgoingfrom said light separating unit and condensing the set of double beamsat a reference diffraction grating, said reference diffraction gratingdisposed at a condensing position of said first condensing means; afirst photodetector for detecting an optical information resulting fromdiffraction by said reference diffraction grating; a second condensingunit for condensing the other set of double beams outgoing from saidlight separating unit; a second photodetector for detecting an opticalinformation resulting from diffraction by a diffraction grating formedon an object to be measured, and located at a condensing position ofsaid second condensing system; and an optical information processingsystem for receiving an output signal of said first photodetector and anoutput signal of said second photodetector to compare the both opticalinformation with each other and for calculating a displacement amount ofsaid object to be measured from a reference position.